github: adapt containerized jobs to be rootless

github: adapt containerized jobs to be rootless

The containerized jobs in GitHub Actions run as root, giving them
special permissions to for example delete files even when the user
shouldn't be able to due to file permissions. This limitation keeps us
from using containerized jobs for most of our Ubuntu-based jobs as it
causes a number of tests to fail.

Adapt the jobs to create a separate user that executes the test suite.
This follows similar infrastructure that we already have in GitLab CI.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>