feat(hc): Force siloed relay tests (#55963)

The tests in the relay job are currently all passing with split silos.
Enforcing this on CI will ensure we don't see regressions here as we
continue to get other jobs passing.

This environment variable effectively turns
`@control_silo_test`/`@region_silo_test` decorators into
`@control_silo_test(stable=True)`/`@region_silo_test(stable=True)`
decorators.

Once all jobs are passing this environment variable will be completely
removed from the codebase.

.github/workflows/backend.yml

@@ -283,6 +283,8 @@ jobs:
           docker ps -a
 
       - name: Run test
+        env:
+          SENTRY_FORCE_SILOED_TESTS: 1
         run: |
           make test-relay-integration