chore: dummy workflow (noir-lang#5438)

# Description

## Problem\*

Adds a dummy workflow for the PR noir-lang#5417

## Summary\*
This will allow the workflow to be run, so that it can be run from the
noir-lang#5417 PR once the dummy version is merged and ran once.


## Additional Context



## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

.github/workflows/mirror-external_libs.yml

@@ -0,0 +1,8 @@
+name: Mirror Repositories
+on:
+  workflow_dispatch: {} 
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo Dummy workflow TODO