ci: install dotnet under the runner home (DOTNET_INSTALL_DIR)

setup-dotnet defaults to /usr/share/dotnet, which the self-hosted
runner user cannot create.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: nficano

.github/workflows/test.yml

@@ -39,6 +39,9 @@ jobs:
       matrix:
         tfm: [net10.0]
     env:
+      # setup-dotnet defaults to /usr/share/dotnet, not writable by the
+      # self-hosted runner user.
+      DOTNET_INSTALL_DIR: /home/app/.dotnet
       DOTNET_NOLOGO: "true"
       DOTNET_CLI_TELEMETRY_OPTOUT: "true"
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: "true"