runcon, chcon: Don't fetch crates at unsupported target

[oech3]

We have *con stubs for workspace build on Windows. But it still fetches useless crates e.g. selinux-sys. We should block them.

Related?: #9968

[oech3]

@drinkcat Would you try to extend this approach for #7397 ?

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will degrade performance by 7.12%

_{Comparing oech3:stub-unfetch (9982802) with main (8d02762)}

Summary

❌ 2 regressed benchmarks
✅ 280 untouched benchmarks
⏩ 38 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	cp_large_file[16]	109.6 KB	118 KB	-7.12%
❌	Memory	dd_copy_partial	127.7 KB	132.1 KB	-3.29%

Footnotes

1. 38 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. ↩