build-gnu.sh: Replace which for portability

[oech3]

which could not be part of core package at some distribution while command is built-in.

[oech3]

Is this not something upstreamable to GNU?

[codspeed]

CodSpeed Performance Report

Merging #9452 will not alter performance

_{Comparing oech3:no-which (72edf3a) with main (89cefb2)}

Summary

✅ 126 untouched
⏩ 6 skipped¹

Footnotes

1. 6 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/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)

[github-actions]

GNU testsuite comparison:

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

[cakebaker]

Thanks!