Auto label PRs that may require extra checks

[celinval]

Description of changes:

Add a new workflow that automatically adds the label "Z-BenchCI" to PRs that touch compiler, toolchain, dependencies and driver files that invoke our dependencies.

I also removed the label from the toolchain update job since this workflow will add the label once the PR is created.

Resolved issues:

#2606

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

Testing:

• How is this change tested? I manually tested in my fork: Change file in the compiler celinval/kani-dev#12

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.