Guard pandoc invocation

Author: megahirt

.github/workflows/build-test-installer-release.yml

@@ -142,7 +142,12 @@ jobs:
           $content = $content -replace '_DATE_', $date
           Set-Content $releaseNotesPath $content -Encoding UTF8
           New-Item -ItemType Directory -Path (Split-Path $outputHtml) -Force | Out-Null
-          pandoc $releaseNotesPath -o $outputHtml --standalone
+          $pandoc = Get-Command pandoc -ErrorAction SilentlyContinue
+          if (-not $pandoc) {
+            Write-Error "pandoc.exe not found on PATH."
+            exit 1
+          }
+          & $pandoc.Path $releaseNotesPath -o $outputHtml --standalone
 
       - name: Build installer
         run: msbuild build\SayMore.proj /t:Installer /p:Configuration=$env:Configuration /p:Version=$env:Version /m