Use `tempfile::TempDir::keep()` instead of deprecated `into_path()` #1439

Job	Run time
build (--test-libcore)	54s
build (--mini-tests)	54s
build (--std-tests)	1m 45s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	2m 3s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	54s
success_gcc12	3s
	6m 33s

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