Gate pull requests on mozjs-sys bump if there are any C++ changes

[delan]

in #595, we made Servo-breaking changes in mozjs-sys without bumping the mozjs-sys crate version, which meant that no new artifact was released for prebuilt mozjs. this broke Servo in servo/servo#38311.

we should prevent pull requests from merging if they have any changes (or at least any C++ changes?) in mozjs-sys without bumping the version. this can be done in build.yml’s integrity job.

(zulip thread, cc @sagudev)

[sagudev]

I am working on this and using integrity job will make thinks harder. Separate workflow is a way to go.

[sagudev]

Okay, so this is tricky we only need to run it on PRs, but we still want it to be part of result that is actually required by MQ, so it should be part of integrity.