Use --locked when building in CI

[Loirooriol]

Thus the build will fail if a PR modifies Cargo.toml but forgets to include the changes in Cargo.lock

[Loirooriol]

See https://doc.rust-lang.org/cargo/commands/cargo-build.html#manifest-options

[Loirooriol]

Works as expected:

• CI failed before landing #19: https://github.com/servo/stylo/actions/runs/8308496963
• CI passes after landing #19: https://github.com/servo/stylo/actions/runs/8309802096