install_pmbootstrap: make pmaports/.git world writeable by all

Works around an issue in the CI after the migration where some dirs are created that are owned by root and not writeable by any other groups/ users.

One way the CI failed from this is .git/refs/remotes was only writeable by root, since root ran git remote add on the freshly cloned repo causing the remotes dir to be created. Later in CI, when other users tried to add remotes, these failed silently (thanks git) and the CI job crashed when it tried to run git fetch on the remote that was never added. At this point in the script, refs/remotes exists so this chmod "fixes" the issue.

I don't have any idea why this is a problem now and not when we were on gitlab.com.