run: Treat `jj run -j0 <command>` as use all available cores #12776

Job	Run time
build (ubuntu-latest)	4m 23s
build (macos-14)	3m 22s
build (windows-latest)	9m 44s
build (ubuntu-latest, --all-features)	5m 20s
Build jj-lib without Git support	41s
cargo-deny (advisories)	22s
Check protos	27s
cargo-deny (bans licenses sources)	21s
Clippy check	2m 44s
Check that MkDocs can build the docs	23s
Check that MkDocs can build the docs with Poetry 1.8	25s
Check formatting	16s
	28m 28s

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