nproc: strip errno

[oech3]

$ target/debug/nproc >/dev/full
nproc: No space left on device
$ nproc >/dev/full
nproc: No space left on device (os error 28)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (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.