Fix Windows tag step: use PowerShell instead of bash

The act runner on Windows doesn't have bash available. Switched back
to PowerShell with the inputs.tag fallback chain. Uses Out-File for
GITHUB_OUTPUT instead of echo redirection.

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

.gitea/workflows/build-sidecar-windows.yml

@@ -16,17 +16,18 @@ jobs:
     steps:
       - name: Determine tag
         id: tag
-        shell: bash
+        shell: powershell
         run: |
-          TAG="${{ inputs.tag }}"
-          if [ -z "$TAG" ]; then
-            TAG="${{ github.event.inputs.tag }}"
-          fi
-          if [ -z "$TAG" ]; then
-            TAG=$(git ls-remote --tags --sort=-v:refname origin 'refs/tags/sidecar-v*' | head -1 | sed 's|.*refs/tags/||')
-          fi
-          echo "Building for tag: ${TAG}"
-          echo "tag=${TAG}" >> $GITHUB_OUTPUT
+          $TAG = "${{ inputs.tag }}"
+          if (-not $TAG) { $TAG = "${{ github.event.inputs.tag }}" }
+          if (-not $TAG) {
+            $remote = git ls-remote --tags origin 'refs/tags/sidecar-v*' 2>$null
+            if ($remote) {
+              $TAG = ($remote -split "`n" | Select-Object -Last 1) -replace '.*refs/tags/', ''
+            }
+          }
+          Write-Host "Building for tag: $TAG"
+          "tag=$TAG" | Out-File -FilePath $env:GITHUB_OUTPUT -Encoding utf8 -Append
 
       - uses: actions/checkout@v4
         with: