ci: add system-tests finished job (#10699)

This job will always run, but will only succeed if the entire workflow
is successful, or fail if any part of the workflow failed

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

.github/workflows/system-tests.yml

@@ -304,3 +304,15 @@ jobs:
         with:
           name: logs_parametric
           path: artifact.tar.gz
+
+  finished:
+    runs-on: ubuntu-latest
+    needs: [parametric, system-tests]
+    if: success() || failure()
+    steps:
+      - name: True when everything else succeeded
+        if: needs.parametric.result == 'success' && needs.system-tests.result == 'success'
+        run: exit 0
+      - name: Fails if anything else failed
+        if: needs.parametric.result != 'success' || needs.system-tests.result != 'success'
+        run: exit 1