build-gnu.sh: correct path suggestion for fetch-gnu.sh

[nutthawit]

This PR fixes an incorrect path suggestion in the util/build-gnu.sh script.

I have updated the suggestion to:

• Adding mkdir -p to ensure the target directory is created.
• Change the fetch-gnu.sh path to util/fetch-gnu.sh.

Fixes #9780

[codspeed-hq]

CodSpeed Performance Report

Merging #9788 will degrade performance by 4.4%

_{Comparing nutthawit:fix-fetch-gnu-path-suggestion (63a6d80) with main (836e5d1)}

Summary

⚡ 1 improvement
❌ 1 regression
✅ 125 untouched
⏩ 6 skipped¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	sort_ascii_c_locale	22.6 ms	21.5 ms	+4.73%
❌	mv_force_overwrite	138 ms	144.3 ms	-4.4%

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:

Congrats! The gnu test tests/printf/printf-surprise is now passing!
Note: The gnu test tests/dd/no-allocate was skipped on 'main' but is now failing.
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[cakebaker]

Thanks for your PR!