github: Use 7z instead of Expand-Archive

7z is included in the list of officially supported/included
applications on the Github Actions Windows images, so we can
rely on it.

This can be significantly faster than the Expand-Archive command,
see e.g. https://github.com/PowerShell/PowerShell/issues/3355.

.github/workflows/test-llvm.yml

@@ -58,7 +58,7 @@ jobs:
           name: windows-ucrt-${{matrix.arch}}-toolchain
       - name: Unpack toolchain
         run: |
-          Expand-Archive llvm-mingw-*.zip -DestinationPath .
+          7z x llvm-mingw-*.zip
           del llvm-mingw-*.zip
           mv llvm-mingw-* c:\llvm-mingw
           echo "c:\llvm-mingw\bin" | Out-File -FilePath $Env:GITHUB_PATH -Encoding utf8 -Append