cp: make --backup and --no-clobber are mutually exclusive

[sylvestre]

Should fix tests/cp/cp-i.sh

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/cp/cp-i is no longer failing!
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cp/cp-i is no longer failing!
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[cakebaker]

Congrats! The gnu test tests/cp/cp-i is no longer failing!

Great :)