[EngSys] Builds not failing if dependencies changed but no "rush update"

[mikeharder]

A couple PRs in the last week changed dependencies but rush update was not run (there were no changes to pnpm-lock.yaml). A recent example is #9082.

What's unexpected is the validation builds for this PR passed. I thought the build should fail if pnpm-lock.yaml does not match the deps in all the package.json files. I thought rush install should always fail in this case, but even if not then the build or tests should fail since the deps would be missing.

Assuming this would have caused a build break in the past, two possible explanations:

1. Something changed in our pipelines, rush, or pnpm which prevents this from breaking the build.
2. It only causes a build break in certain cases, say if a dependency version is updated or a totally new dependency is added. But if all the dependencies are already used by other packages, it doesn't cause any failures.

[mikeharder]

Dup of #11789