gh-27: Fix error in GH Actions workflow.

Author: python-processing-unit

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Build interpreter and extensions (Developer PowerShell)
         shell: powershell
         run: |
-          $vswhere = Join-Path $env:'ProgramFiles(x86)' 'Microsoft Visual Studio\Installer\vswhere.exe'
+          $vswhere = Join-Path ${env:ProgramFiles(x86)} 'Microsoft Visual Studio\Installer\vswhere.exe'
           if (-not (Test-Path $vswhere)) { Write-Error "vswhere not found at $vswhere"; exit 1 }
           $inst = & $vswhere -latest -products * -requires Microsoft.Component.MSBuild -property installationPath
           if (-not $inst) { Write-Error 'Visual Studio installation not found'; exit 1 }
@@ -29,7 +29,7 @@ jobs:
       - name: Run tests (Developer PowerShell)
         shell: powershell
         run: |
-          $vswhere = Join-Path $env:'ProgramFiles(x86)' 'Microsoft Visual Studio\Installer\vswhere.exe'
+          $vswhere = Join-Path ${env:ProgramFiles(x86)} 'Microsoft Visual Studio\Installer\vswhere.exe'
           if (-not (Test-Path $vswhere)) { Write-Error "vswhere not found at $vswhere"; exit 1 }
           $inst = & $vswhere -latest -products * -requires Microsoft.Component.MSBuild -property installationPath
           if (-not $inst) { Write-Error 'Visual Studio installation not found'; exit 1 }