Pin stack2cabal version used by scripts/generate-cabal-files.sh

Stack uses the most recent available version by default, and a new stack2cabal version has been released recently with a different output format. Pin stack2cabal version to make sure it will not change automatically again

By the way, it would be a good idea to update resolver in snapshot-stack2cabal.yaml to match the one in stack.yaml, to reduce rebuilds, but unfortunately stack2cabal fails to build with lts-15