test__custom_commit_message acceptance test intermittently fails

[amimas]

The acceptance test located at tests/acceptance/standard/test_files.py::TestFiles::test__custom_commit_message intermittently fails. Examples:

• https://github.com/gitlabform/gitlabform/actions/runs/6031288955/job/16364708681
• https://github.com/gitlabform/gitlabform/actions/runs/6072962483/job/16473939160

Processing file 'README.md' in branch 'gitlabform_branch_liqueur'
----------------------------- Captured stderr call -----------------------------
Error: Operation add on file README.md in branch gitlabform_branch_liqueur not permitted, but we don't have a branch protection configuration provided for this branch. Breaking as we cannot unprotect the branch as we would not know how to protect it again. 1
------------------------------ Captured log call -------------------------------
...
=========================== short test summary info ============================
FAILED tests/acceptance/standard/test_files.py::TestFiles::test__custom_commit_message - SystemExit: 1
============== 1 failed, 106 passed, 3 rerun in 255.63s (0:04:15) ==============

It'd be nice to stabilize the test. Otherwise processing PRs require extra time.