ci: use a fake key for system tests [backport #6702 to 1.16] (#6705)

Backport of #6702 to 1.16

System tests are mandatory to merge PR. The issue is that system tests
requires an API_KEY stored in secrests, and it's not possible to share
secrets when the PR comes from a forks. Hacks exists, but none of them
is fully satisfaying.

Turns out that this key does not have to be a valid key for most
scenario, as long as nothing is tested on the backend.

So trying to use a fake key.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] 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)

Co-authored-by: Charles de Beauchesne <[email protected]>

Author: P403n1x87

.github/workflows/system-tests.yml

@@ -19,7 +19,10 @@ jobs:
     env:
       TEST_LIBRARY: python
       WEBLOG_VARIANT: ${{ matrix.weblog-variant }}
-      DD_API_KEY: ${{ secrets.DD_API_KEY }}
+      # system-tests requires an API_KEY, but it does not have to be a valid key, as long as we don't run a scenario
+      # that make assertion on backend data. Using a fake key allow to run system tests on PR originating from forks.
+      # If ever it's needed, a valid key exists in the repo, using ${{ secrets.DD_API_KEY }}
+      DD_API_KEY: 1234567890abcdef1234567890abcdef
     steps:
       - name: Setup python 3.9
         uses: actions/setup-python@v4