Cannot run Flink tests in Azure Pipeline - S3 credentials

[ta...@gmail.com]

Hi dev,

I have test failures irrelevant to my modifications (
https://issues.apache.org/jira/browse/FLINK-23972), but very annoying.
Tests fail because they cannot properly use S3 credentials. Could someone
help me to make those tests running?

Examples:
https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=24748&view=logs&j=c88eea3b-64a0-564d-0031-9fdcd7b8abee&t=070ff179-953e-5bda-71fa-d6599415701c&l=11859
https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=24748&view=logs&j=fc5181b0-e452-5c8f-68de-1097947f6483&t=995c650b-6573-581c-9ce6-7ad4cc038461&l=29552

If we check the URIs, we see the following:
s3://$(SECRET_S3_BUCKET)/temp/test_file_sink-d402aca3-66e9-4faf-ae27-f5ba8556606b-chk
s3n://$(SECRET_S3_BUCKET)/temp/tests-9e728f65-14c6-4fa2-aa4d-9e61e994efe0

In other words, the S3-related variables (declared in the Azure Pipeline
definition build-apache-repo.yaml) are not properly expanded.
This is annoying because:
1. I cannot finish my PR - some tests irrelevant to my modifications are
failing.
2. The S3-related tests start, despite there are no required credentials.
In the output of test_file_sink.sh we can see:
   Found AWS bucket $(SECRET_S3_BUCKET), running the e2e test.
Compare with the output from a passing test (with access to S3 credentials):
Found AWS bucket ***, running the e2e test.
This is quite misleading behavior. It would be better to skip the S3 tests
with a corresponding message on the stderr.

The other possible way for me to progress on my PR is to ignore the
failures of S3-related tests. However, it smells...

Arseniy Tashoyan