file sink e2e tests with local setup are not executed if s3 credentials are not provided

While looking into test parity between Azure Pipelines and GitHub Actions, I noticed that the only OpenSSL-based test is flink-end-to-end-tests/test-scripts/test_file_sink.sh which comes with a local and a s3 setting.

S3 requires S3 credential and the bucket information to be available through environment variables. That's handled in flink-end-to-end-tests/test-scripts/common_s3.sh#L25. The problem is that this shell script is also source'd when running the test with local setup (see flink-end-to-end-tests/test-scripts/test_file_sink.sh#L27).

This means that also the local test is only running in the main repository which is not necessary.