fix(ci): use Windows PowerShell 5.1 (powershell) instead of pwsh

The self-hosted Windows runner runs as LocalSystem and only sees the
machine PATH. winget-installed pwsh lands in the user PATH and is
invisible to the service. Built-in Windows PowerShell 5.1 (`powershell`)
handles the same syntax we use, so switch all steps to it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Debug — show what checkout produced
-        shell: pwsh
+        shell: powershell
         run: |
           Write-Host "CWD: $(Get-Location)"
           Write-Host "GITHUB_WORKSPACE: $env:GITHUB_WORKSPACE"
@@ -74,7 +74,7 @@ jobs:
 
       - name: Resolve mage.exe + signtool.exe paths
         id: tools
-        shell: pwsh
+        shell: powershell
         run: |
           # mage.exe lives under .NET 4.8 Developer Pack. Find any installed copy.
           $mage = Get-ChildItem 'C:\Program Files (x86)\Microsoft SDKs\Windows' -Recurse -Filter mage.exe -ErrorAction SilentlyContinue |
@@ -95,7 +95,7 @@ jobs:
 
       - name: Compute version from tag
         id: ver
-        shell: pwsh
+        shell: powershell
         run: |
           $tag = "${{ github.ref_name }}"
           if ($tag -notmatch '^v(\d+\.\d+\.\d+)$') {
@@ -108,7 +108,7 @@ jobs:
           Write-Host "Version: $version"
 
       - name: Ensure code-signing cert in LocalMachine\My
-        shell: pwsh
+        shell: powershell
         env:
           THUMB:    ${{ secrets.CODESIGN_THUMBPRINT }}
           PFX_B64:  ${{ secrets.CODESIGN_CERT_PFX_BASE64 }}
@@ -142,7 +142,7 @@ jobs:
       # picks it by thumbprint, the service runs as LocalSystem so the runtime
       # search reaches both stores.
       - name: Sign assemblies
-        shell: pwsh
+        shell: powershell
         run: |
           $thumb = "${{ secrets.CODESIGN_THUMBPRINT }}"
           & "$env:SIGNTOOL" sign /sha1 $thumb /fd SHA256 /tr http://timestamp.digicert.com /td SHA256 `
@@ -152,7 +152,7 @@ jobs:
             src/OutlookAddinProtocolHandler/bin/Release/net48/OutlookAddinProtocolHandler.exe
 
       - name: Publish ClickOnce
-        shell: pwsh
+        shell: powershell
         run: |
           $version       = "${{ steps.ver.outputs.version }}"
           $versionUscore = "${{ steps.ver.outputs.versionUscore }}"
@@ -195,7 +195,7 @@ jobs:
               ForEach-Object { Rename-Item -LiteralPath $_.FullName -NewName ($_.Name + '.deploy') }
 
       - name: Stage protocol handler
-        shell: pwsh
+        shell: powershell
         run: |
           $stagingDir = "publish/protocol-handler"
           New-Item -ItemType Directory -Force -Path $stagingDir | Out-Null