build-gnu.sh: adapt basenc message to clap changes

[cakebaker]

The "unexpected argument" message from clap has changed slightly. This PR updates the message for the basenc test, which should allow basenc.pl to pass.

[github-actions]

GNU testsuite comparison:

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

[sylvestre]

Congrats! The gnu test tests/basenc/basenc is no longer failing!

[sylvestre]

wahou, nice :)