build

build: add `rust-toolchain.toml` #14816

Job	Run time
cargo-deny (advisories)	39s
Build jj-lib without Git support	54s
cargo-deny (bans licenses sources)	41s
build (ubuntu-latest)	4m 29s
Check protos	59s
Clippy check	2m 38s
Check that MkDocs can build the docs	26s
build (macos-13)	5m 58s
Check that MkDocs can build the docs with Poetry 1.8	32s
build (macos-14)	3m 51s
Check formatting	18s
build (windows-latest)	11m 54s
build (ubuntu-latest, --all-features)	5m 44s
	39m 3s

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