fix(ci): unset DOTNET_ROOT for Pester steps, add 15min timeout

The setup-dotnet action sets DOTNET_ROOT which overrides PS 7.x's
bundled .NET runtime for assembly resolution, causing System.Runtime
9.0.0.0 FileNotFoundException. Unsetting it for Pester steps lets PS
use its own runtime.

Also adds timeout-minutes: 15 to prevent runaway jobs from consuming
Actions minutes (PS 5.1 job was hanging for 5+ hours).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/integration-tests.yml

@@ -201,6 +201,8 @@ jobs:
       - name: Run integration tests
         shell: pwsh
         env:
+          DOTNET_ROOT: ''
+          DOTNET_MULTILEVEL_LOOKUP: ''
           PVETEST_HOST: ${{ steps.target.outputs.host }}
           PVETEST_PORT: ${{ steps.target.outputs.port }}
           PVETEST_APITOKEN: ${{ steps.target.outputs.token }}