numfmt: apply prefix after scaling for --to-unit

[BipulLamsal]

Fixes #11666

Prefix selection was done before scaling, which caused incorrect results while using --to-unit.

This change moves prefix selection to occur after scaling, ensuring correct IEC/SI formatting.

Tested with:

./target/debug/numfmt --to=iec-i --to-unit=885 100000 => 113
./target/debug/numfmt --to=iec-i --to-unit=885 1000000 => 1.2Ki

[sylvestre]

please also remove the ignore for the test in 53c7d6f

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!

[cakebaker]

Thanks for your PR!