ci: allow for using the latest build of the experimental installer

Until these repos are potentially merged, this is good for dogfooding
alongside the experimental installer

More immediately though, we need a patch for the experimental installer
to really work in CI at all, and that hasn't landed in a tag yet. So,
this lets us use it now!

Author: getchoo

.github/actions/install-nix-action/action.yaml

@@ -7,6 +7,10 @@ inputs:
   experimental-installer:
     description: "Whether to use the experimental installer to install Nix"
     default: false
+  experimental-installer-version:
+    description: "Version of the experimental installer to use. If `latest`, the newest artifact from the default branch is used."
+    # TODO: This should probably be pinned to a release after https://github.com/NixOS/experimental-nix-installer/pull/49 lands in one
+    default: "latest"
   extra_nix_config:
     description: "Gets appended to `/etc/nix/nix.conf` if passed."
   install_url:
@@ -50,36 +54,45 @@ runs:
       env:
         GH_TOKEN: ${{ inputs.github_token }}
         DOGFOOD_REPO: "NixOS/nix"
-    - name: "Download experimental installer"
+    - name: "Gather system info for experimental installer"
       shell: bash
-      id: download-experimental-nix-installer
       if: ${{ inputs.experimental-installer == 'true' }}
       run: |
+        echo "::notice Using experimental installer from $EXPERIMENTAL_INSTALLER_REPO (https://github.com/$EXPERIMENTAL_INSTALLER_REPO)"
+
         if [ "$RUNNER_OS" == "Linux" ]; then
-          INSTALLER_OS="linux"
+          echo "EXPERIMENTAL_INSTALLER_SYSTEM=linux" >> "$GITHUB_ENV"
         elif [ "$RUNNER_OS" == "macOS" ]; then
-          INSTALLER_OS="darwin"
+          echo "EXPERIMENTAL_INSTALLER_SYSTEM=darwin" >> "$GITHUB_ENV"
         else
           echo "::error ::Unsupported RUNNER_OS: $RUNNER_OS"
+          exit 1
         fi
 
         if [ "$RUNNER_ARCH" == "X64" ]; then
-          INSTALLER_ARCH="x86_64"
+          echo "EXPERIMENTAL_INSTALLER_ARCH=x86_64" >> "$GITHUB_ENV"
         elif [ "$RUNNER_ARCH" == "ARM64" ]; then
-          INSTALLER_ARCH="aarch64"
+          echo "EXPERIMENTAL_INSTALLER_ARCH=aarch64" >> "$GITHUB_ENV"
         else
           echo "::error ::Unsupported RUNNER_ARCH: $RUNNER_ARCH"
+          exit 1
         fi
 
-        EXPERIMENTAL_INSTALLER_ARTIFACT="nix-installer-$INSTALLER_ARCH-$INSTALLER_OS"
-        EXPERIMENTAL_INSTALLER_PATH="$GITHUB_WORKSPACE/$EXPERIMENTAL_INSTALLER_ARTIFACT"
-        # TODO: This uses the latest release. It should probably be pinned, or dogfood the experimental repo's default branch - similar to the above
-        gh release download -R "$EXPERIMENTAL_INSTALLER_REPO" -D "$EXPERIMENTAL_INSTALLER_PATH" -p "nix-installer.sh" -p "$EXPERIMENTAL_INSTALLER_ARTIFACT"
-        chmod +x "$EXPERIMENTAL_INSTALLER_PATH/$EXPERIMENTAL_INSTALLER_ARTIFACT"
+        echo "EXPERIMENTAL_INSTALLER_ARTIFACT=nix-installer-$EXPERIMENTAL_INSTALLER_ARCH-$EXPERIMENTAL_INSTALLER_SYSTEM" >> "$GITHUB_ENV"
+      env:
+        EXPERIMENTAL_INSTALLER_REPO: "NixOS/experimental-nix-installer"
+    - name: "Download latest experimental installer"
+      shell: bash
+      id: download-latest-experimental-installer
+      if: ${{ inputs.experimental-installer == 'true' && inputs.experimental-installer-version == 'latest' }}
+      run: |
+        RUN_ID=$(gh run list --repo "$EXPERIMENTAL_INSTALLER_REPO" --workflow ci.yml --branch main --status success --json databaseId --jq ".[0].databaseId")
 
-        echo "installer-path=$EXPERIMENTAL_INSTALLER_PATH" >> "$GITHUB_OUTPUT"
+        EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR="$GITHUB_WORKSPACE/$EXPERIMENTAL_INSTALLER_ARTIFACT"
+        mkdir -p "$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR"
 
-        echo "::notice Using experimental installer from $EXPERIMENTAL_INSTALLER_REPO (https://github.com/$EXPERIMENTAL_INSTALLER_REPO)"
+        gh run download "$RUN_ID" --repo "$EXPERIMENTAL_INSTALLER_REPO" -n "$EXPERIMENTAL_INSTALLER_ARTIFACT" -D "$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR"
+        echo "installer-path=file://$EXPERIMENTAL_INSTALLER_DOWNLOAD_DIR" >> "$GITHUB_OUTPUT"
       env:
         GH_TOKEN: ${{ inputs.github_token }}
         EXPERIMENTAL_INSTALLER_REPO: "NixOS/experimental-nix-installer"
@@ -94,6 +107,7 @@ runs:
       if: ${{ inputs.experimental-installer == 'true' }}
       with:
         diagnostic-endpoint: ""
-        local-root: ${{ steps.download-experimental-nix-installer.outputs.installer-path }}
+        # TODO: It'd be nice to use `artifacts.nixos.org` for both of these, maybe through an `/experimental-installer/latest` endpoint? or `/commit/<hash>`?
+        source-url: ${{ inputs.experimental-installer-version == 'latest' && steps.download-latest-experimental-installer.outputs.installer-path || 'https://artifacts.nixos.org/experimental-installer/tag/${{ inputs.experimental-installer-version }}/${{ env.EXPERIMENTAL_INSTALLER_ARTIFACT }}' }}
         nix-package-url: ${{ inputs.dogfood == 'true' && steps.download-nix-installer.outputs.tarball-path || (inputs.tarball_url || '') }}
         extra-conf: ${{ inputs.extra_nix_config }}