build

Implement hacky variant of `jj purge` #13519

Job	Run time
build (ubuntu-latest)	4m 34s
build (macos-13)	6m 32s
Build jj-lib without Git support	45s
build (macos-14)	3m 22s
build (windows-latest)	9m 16s
cargo-deny (advisories)	23s
Check protos	33s
build (ubuntu-latest, --all-features)	5m 25s
cargo-deny (bans licenses sources)	24s
Check that MkDocs can build the docs	19s
Check that MkDocs can build the docs with Poetry 1.8	20s
Check formatting	17s
Clippy check	2m 22s
	34m 32s

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