Use C: for building in CI

According to the issue tracker this has more free space than D: now

.github/workflows/build.yml

@@ -61,6 +61,7 @@ jobs:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |
         $env:PACKAGER='CI (msys2-devtools/' + $env:GITHUB_SHA.Substring(0, 8) + '/' + $env:GITHUB_RUN_ID + ')'
-        $BUILD_ROOT=((Get-Location).Drive.Root) + '_'
+        $BUILD_ROOT='C:\_'
         $MSYS2_ROOT=(msys2 -c 'cygpath -w /')
+        Get-PSDrive -PSProvider FileSystem
         python autobuild.py build "$MSYS2_ROOT" "$BUILD_ROOT"