Refactor run_custom_user_commands

[codefiles]

With #2026, the issue was misdiagnosed.

On the pathlib slash operator:

If the argument is an absolute path, the previous path is ignored.

Source: https://docs.python.org/3/library/pathlib.html#operators

Example of the problem, script_path as an absolute path:

>>> from pathlib import Path
>>> target = Path('/mnt')
>>> script_path = '/var/tmp/user-command.sh'
>>> target / script_path
PosixPath('/var/tmp/user-command.sh')

This fails to produce the intended /mnt/var/tmp/user-command.sh.

script_path as a relative path (without the / prefix):

>>> from pathlib import Path
>>> target = Path('/mnt')
>>> script_path = 'var/tmp/user-command.sh'
>>> target / script_path
PosixPath('/mnt/var/tmp/user-command.sh')