Allow the main workflow to be called in a distant workflow

[cbeauchesne]

Motivation

Allow DataDog/system-tests/.github/workflows/system-tests.yml to be called from a different repo

Changes

• checkout -> force system-tests repo to be checkout, rather than the calling repo

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner. We're working on refining the codeowners file quickly.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Relevant label (run-parametric-scenario, run-profiling-scenario...) are presents
• No system-tests internal is modified. Otherwise, I have the approval from R&P team
• CI is green, or failing jobs are not related to this change (and you are 100% sure about this statement)
• A docker base image is modified?
  • the relevant build-XXX-image label is present
  • To R&P team: locally build and push the image to hub.docker.com
• A scenario is added (or removed)?
  • Get a review from R&P team
  • Once merged, add (or remove) it in system-test-dasboard nightly