CONTRIBUTING.md: Add URL to get latest GNU coreutils

[oech3]

This PR adds guide to get latest stable version of GNU coreutils for bug reporters (with some warnings).

packager of GNU coreutils is @BAMF0

[codspeed-hq]

Merging this PR will improve performance by 4.88%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	sort_ascii_utf8_locale	16.1 ms	15.4 ms	+4.88%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:patch-4 (453f76f) with main (77933ae)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (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/pid-pipe (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!

[collinfunk]

I'm not sure why Ubuntu has an older GNU coreutils package than uutils. It seems like poor user interface to me.

Given that you cannot switch to GNU coreutils, despite promises otherwise, because it would break build-essential, I guess it is too be expected...

[oech3]

I think they are intentionally masking improvement of GNU by using legacy version to encourage usage of rust-coreutils.