MacOS App: make only files read-only (#38289)
PR #38235 seemed to cause problems:
- it seems to break GitHub Actions: https://github.com/JuliaParallel/MPI.jl/runs/1345656760?check_suite_focus=true#step:4:33
- it now requires a password to upgrade a patch release (which replace the `/Applications/Julia-1.6.app` directory)
This makes only the files read-only, which should solve the problem
