Bump install-nix-action to v12

Author: adisbladis

.github/workflows/ci.yaml

@@ -11,7 +11,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
       - name: Nix
-        uses: cachix/install-nix-action@v8
+        uses: cachix/install-nix-action@v12
       - name: Build
         run: 'nix-build -I nixpkgs=channel:nixos-unstable --quiet release.nix -A nixops-aws.x86_64-linux --show-trace'
   # Run black v19.10b0 and ensure no diff, using a pinned channel to ensure the
@@ -25,24 +25,30 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
       - name: Nix
-        uses: cachix/install-nix-action@v8
+        uses: cachix/install-nix-action@v12
       - name: Black
         run: 'nix-shell ./shell.nix --run "black --check ."'
+        env:
+          NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"
   flake8:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v2
       - name: Nix
-        uses: cachix/install-nix-action@v8
+        uses: cachix/install-nix-action@v12
       - name: Flake8
         run: 'nix-shell ./shell.nix --run "flake8 nixops_aws"'
+        env:
+          NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"
   mypy:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v2
       - name: Nix
-        uses: cachix/install-nix-action@v8
+        uses: cachix/install-nix-action@v12
       - name: Mypy
         run: 'nix-shell ./shell.nix --run "mypy nixops_aws"'
+        env:
+          NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"