ci: drop DOTNET_INSTALL_DIR (runner context invalid in job env)

runner.temp is not available when evaluating job-level env, causing a
workflow startup failure. setup-dotnet installs to the default writable
location on GitHub-hosted runners, so the override is unnecessary.

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

Author: nficano

.github/workflows/test.yml

@@ -42,9 +42,6 @@ jobs:
       fail-fast: false
       matrix:
         tfm: [net10.0]
-    env:
-      # Install the .NET SDK into a runner-writable directory.
-      DOTNET_INSTALL_DIR: ${{ runner.temp }}/dotnet
 
     steps:
       - name: Checkout