ci: disable credential persistence in checkout steps

[xtqqczze]

Follow-up #6975

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/tail/pid-pipe is no longer failing!

[codspeed-hq]

Merging this PR will degrade performance by 23.82%

❌ 1 regressed benchmark
✅ 308 untouched benchmarks
⏩ 46 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	cp_recursive_deep_tree[(120, 4)]	532.7 KB	699.2 KB	-23.82%

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_{Comparing xtqqczze:ci-persist-credentials (32f590d) with main (fe7e262)}

Footnotes

1. 46 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. ↩

[cakebaker]

Thanks!