GnuTests: remove LD_PRELOAD from nproc-quota.sh & avoid 0 period

[oech3]

backport coreutils/coreutils#309

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes 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/seq/seq-epipe is now being skipped but was previously passing.

[oech3]

rebased for coreutils/coreutils#310

[codspeed-hq]

Merging this PR will improve performance by 4.84%

⚡ 1 improved benchmark
✅ 330 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	sort_ascii_utf8_locale	16.1 ms	15.4 ms	+4.84%

Tip

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_{Comparing oech3:patch-5 (3d24c0c) with main (2a92fcc)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩