ci: only run `cargo-shear` when Rust changes #8402

Job	Run time
Conformance	2m 25s
Cargo Deny	4s
Doc	45s
Check and Build (windows-latest)	1m 18s
Check and Build (ubuntu-latest)	35s
Format	12s
Clippy	48s
Check and Build (macos-14)	43s
Test (windows-latest)	2m 4s
Test (ubuntu-latest)	1m 9s
Spell Check	4s
Test (macos-14)	1m 1s
Check Unused Dependencies	4s
Check Wasm	1m 27s
	12m 39s

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