mktemp: remove duplicate code from uumain

[cakebaker]

This PR removes some duplicate code from uumain and calls mktemp() instead, a function we provide for nushell and wasn't used in our code base so far.

[sylvestre]

ouch, thanks :)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/mktemp/mktemp. tests/mktemp/mktemp is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!