CI: Purge disk space at background to avoid delaying

[oech3]

rm -rf /usr/share/dotnet /usr/local/lib/android takes 50 sec+. Better to do that at background at early stage.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/follow-name. tests/tail/follow-name is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 7.93%

_{Comparing oech3:purge-space-bg (2769065) with main (0b58260)}

Summary

⚡ 1 improved benchmark
✅ 281 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	sort_ascii_utf8_locale	6.7 MB	6.2 MB	+7.93%

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. ↩