Xcode 12 breaks OCaml builds prior 4.08

GH Actions switched to Xcode 12: https://github.com/actions/virtual-environments/issues/1712
Which fails OCaml builds prior 4.08: https://github.com/ocaml/opam/issues/4364

I currently use sudo xcode-select -s "/Applications/Xcode_11.7.app" workaround to build PPXs.