ci: Cabal install packdeps outside of project dir

Invoking cabal in the project directory with a cabal.project file might
fail on unrelated dependencies missing, see
4f539734be

Doesn't fail in this repo, but let's keep the workflows in sync.

.github/workflows/packdeps.yml

@@ -23,6 +23,7 @@ jobs:
         run: |
           set -ex
           echo "$HOME/.cabal/bin" >> $GITHUB_PATH
+          cd  # go somewhere without a cabal.project
           cabal install packdeps
       - name: Check package bounds (all)
         continue-on-error: true