build-gnu.sh: refactor & dedup bins

[oech3]

b3sum is not GNU.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[codspeed]

CodSpeed Performance Report

Merging #9295 will improve performances by 2.23%

_{Comparing oech3:patch-2 (d6bc9ea) with main (63ee648)}

Summary

⚡ 1 improvement
✅ 123 untouched
⏩ 5 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	du_max_depth_balanced_tree[(6, 4, 10)]	33.1 ms	32.4 ms	+2.23%

Footnotes

1. 5 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. ↩