build-gnu.sh: Use system tools

[oech3]

Avoid accidentaly swapping readlink with our binary.

[github-actions]

GNU testsuite comparison:

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

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails 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)

[codspeed-hq]

CodSpeed Performance Report

Merging #9251 will not alter performance

_{Comparing oech3:prefer-g (44ae621) with main (d432131)}

Summary

✅ 123 untouched
⏩ 5 skipped¹

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. ↩

[github-actions]

GNU testsuite comparison:

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

[github-actions]

GNU testsuite comparison:

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

[cakebaker]

Nice refactoring, thanks!

[oech3]

@cakebaker Not a very important cleanup, but waiting for #9263 too.