Add `flock` shim #8326

Job	Run time
style checks	2m 37s
build (ubuntu-latest, x86_64-unknown-linux-gnu)	23m 13s
build (macos-14, aarch64-apple-darwin)	19m 32s
build (windows-latest, i686-pc-windows-msvc)	17m 38s
cronjob failure notification	0s
bors build finished	0s
bors build finished	0s
	1h 3m 0s

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