Fix stability of reproducible tarballs from git repos

[lexming]

Following some test failures with our build bots (see easybuilders/easybuild-easyconfigs#19770), I found out that the tarballs of git repos are not fully reproducible as they dependent on some local settings of the host system:

1. resetting file modification times with touch -t depends on the local timezone
2. some file permissions of the cloned repository depend on default directory permissions

This PR fixes both issues:

1. reset file modification times with touch --date and using unix epoch zero
2. always remove write permissions from group and others (follows recommendation in https://www.gnu.org/software/tar/manual/html_node/Reproducibility.html)

[boegel]

lgtm