ci: write the Windows steps in PowerShell, not bash

The core-only fallback and the "Show what was built" step were written with
`shell: bash`. `winvm-builder` is a plain Windows VM -- per this repo's own
history it does not even have cmake preinstalled -- so bash cannot be assumed
present, and those two steps would have failed on the shell rather than on
anything real. Both are PowerShell now, using $LASTEXITCODE and Test-Path.

Caught by reading the workflow back rather than by a CI run: Windows is
serialised behind a long queue and would not have surfaced this for another
hour.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

.gitea/workflows/build.yml

@@ -122,13 +122,16 @@ jobs:
         #
         # Same fallback as macOS, and the same warning: a green job that
         # quietly stopped building the plugin is worse than a red one.
-        shell: bash
+        # PowerShell, not bash: this runner is a plain Windows VM and bash
+        # cannot be assumed present.
         run: |
-          if ! cmake -S . -B build -A x64; then
-            echo "::warning::OBS SDK bootstrap failed on Windows; building the core library only. The plugin module was NOT built."
-            rm -rf build
+          cmake -S . -B build -A x64
+          if ($LASTEXITCODE -ne 0) {
+            Write-Host "::warning::OBS SDK bootstrap failed on Windows; building the core library only. The plugin module was NOT built."
+            Remove-Item -Recurse -Force build -ErrorAction SilentlyContinue
             cmake -S . -B build -A x64 -DSTPLUGIN_BOOTSTRAP_OBS=OFF
-          fi
+            if ($LASTEXITCODE -ne 0) { exit 1 }
+          }
 
       - name: Build
         run: cmake --build build --config Release
@@ -137,8 +140,12 @@ jobs:
         run: ctest --test-dir build -C Release --output-on-failure
 
       - name: Show what was built
-        shell: bash
-        run: ls -la build/package/bin || echo "no plugin module was built (core library only)"
+        run: |
+          if (Test-Path build\package\bin) {
+            Get-ChildItem build\package\bin
+          } else {
+            Write-Host "no plugin module was built (core library only)"
+          }
 
       - name: Upload plugin
         continue-on-error: true