chore(ci): use dd-sts for system-tests test optimization (#55)

## Summary

Migrates system-tests CI to use
[dd-sts](https://github.com/DataDog/dd-sts-action) for Datadog Test
Optimization instead of long-lived API keys.

All repositories now share a single `system-tests` policy (see
[dd-source#408172](DataDog/dd-source#408172)) —
no per-repo policy is needed.

Depends on
[DataDog/system-tests#6726](DataDog/system-tests#6726).

### Changes
- Pin system-tests to `1e5d6b709` (current `main`, pre-migration) to
allow a controlled rollout: repos stay on the pre-migration workflow
until their pin is explicitly updated to the post-merge SHA

Author: nccatoni

.github/workflows/system-tests.yml

@@ -43,16 +43,16 @@ jobs:
   main:
     needs:
       - build-artifacts
-    uses: DataDog/system-tests/.github/workflows/system-tests.yml@main
+    uses: DataDog/system-tests/.github/workflows/system-tests.yml@1e5d6b7096279ca43ce4826fda3cc805635b63c1
     secrets:
       TEST_OPTIMIZATION_API_KEY: ${{ secrets.DD_API_KEY_CI_APP }}
       DD_API_KEY: ${{ secrets.DD_API_KEY_CI_APP }}
     permissions:
       contents: read
       id-token: write
-      packages: write
     with:
       library: cpp_httpd
+      ref: 1e5d6b7096279ca43ce4826fda3cc805635b63c1
       binaries_artifact: system_tests_binaries
       desired_execution_time: 300  # 5 minutes
       scenarios_groups: tracer-release