ci: install .NET into runner-writable dir (fix /home/app leftover on GitHub-hosted runner)

The 2026-06-16 migration to GitHub-hosted ubuntu-24.04-arm left the
self-hosted DOTNET_INSTALL_DIR=/home/app/.dotnet, which is not writable
on hosted runners, so setup-dotnet failed at 'mkdir /home/app'. Point it
at ${{ runner.temp }}/dotnet instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nficano

.github/workflows/test.yml

@@ -39,9 +39,8 @@ 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
+      # Install the .NET SDK into a runner-writable directory.
+      DOTNET_INSTALL_DIR: ${{ runner.temp }}/dotnet
       DOTNET_NOLOGO: "true"
       DOTNET_CLI_TELEMETRY_OPTOUT: "true"
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: "true"