mktemp: strip permission denied system error code

[Devel08]

closes #13720, pr #13815 seems to be abondoned

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
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/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.

[Devel08]

well, -p / wouldn't work on some runners as its read-only fs

[Devel08]

read-only fs error would contain an os error too, we can just check if it doesnt

[codspeed-hq]

Merging this PR will degrade performance by 3.17%

❌ 1 regressed benchmark
✅ 284 untouched benchmarks
⏩ 108 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	numfmt_to_si_precision[10000]	92.5 ms	95.5 ms	-3.17%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing Devel08:fix-issue-13720 (32d48f7) with main (acf5747)²}

Footnotes

1. 108 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. ↩

2. No successful run was found on main (8ba86de) during the generation of this report, so acf5747 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩