Pin system-tests to SHA with pinned download-artifact action

Update DataDog/system-tests ref from 61f7ee29e7432cba821a4d303c95929160bd23fd
to ab6ccde3946ded935e7b1ffddf260b982275795e which pins actions/download-artifact
to a full commit SHA, fixing the enterprise policy violation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jandro996

.github/workflows/run-system-tests.yaml

@@ -62,7 +62,7 @@ jobs:
     needs:
     - build
     # If you change the following comment, update the pattern in the update_system_test_reference.sh script to match.
-    uses: DataDog/system-tests/.github/workflows/system-tests.yml@61f7ee29e7432cba821a4d303c95929160bd23fd # system tests are pinned for releases only
+    uses: DataDog/system-tests/.github/workflows/system-tests.yml@ab6ccde3946ded935e7b1ffddf260b982275795e # system tests are pinned for releases only
     secrets:
       TEST_OPTIMIZATION_API_KEY: ${{ secrets.DATADOG_API_KEY_PROD }}
     permissions:
@@ -72,7 +72,7 @@ jobs:
     with:
       library: java
        # If you change the following comment, update the pattern in the update_system_test_reference.sh script to match.
-      ref: 61f7ee29e7432cba821a4d303c95929160bd23fd # system tests are pinned for releases only
+      ref: ab6ccde3946ded935e7b1ffddf260b982275795e # system tests are pinned for releases only
       binaries_artifact: binaries
       desired_execution_time: 900  # 15 minutes
       scenarios_groups: tracer-release