:bug: Integration test proto build robustness

[evaline-ju]

Describe the bug

Sometimes the first CI integration run on PRs will fail on envoy.service import, suggesting a proto build failure. Currently, there's no extra import checks before integration tests to not double imports that are already invoked through integration tests.

Steps To Reproduce

Open PRs, triggering integration tests

Expected Behavior

Integration tests can run with expected proto build and imports.

Additional Context

I had attempted to cache proto builds. Either the build itself or the cache uses are not particularly dependable. Re-runs (second attempts) tend to pass integration tests.