Update README.md with new chown command

[yforster]

As discussed on Zulip, at least for MetaCoq we need to fix permissions already before the installation, otherwise installing dependencies via opam fails.

[yforster]

The sentence This is not an issue when relying on opam to build the Coq project. in the README might be incorrect with the newest git version. To me, it might be reasonable to run the chown by default in before_install.

[erikmd]

Thanks @yforster!

Actually I won't be able to merge your patch because:

• before_install already has a value that should be kept;
• merely tweaking before_install won't be enough because the following step would be missing:

  - name: Revert permissions
    # to avoid a warning at cleanup time
    if: ${{ always() }}
    run: sudo chown -R 1001:116 .  # <--

So I believe I'll come up with a (pre-entrypoint, post-entrypoint)-based solution