Fix conflict with AWS credentials

We've added support for S3 with custom endpoints and added a test here https://github.com/feast-dev/feast/blob/master/sdk/python/tests/integration/offline_store/test_s3_custom_endpoint.py#L13

Currently our test suite sets it's own AWS credentials [here](https://github.com/feast-dev/feast/blob/a48bcd0d9ca27117c42ecb5759189786c521322c/.github/workflows/integration_tests.yml#L47). These credentials will conflict with the ones that are necessary for the S3 endpoint test, which uses Minio, given that all the tests will execute in the same process.

For the time being we have disabled the test (`test_s3_custom_endpoint`). We should make it possible to run the test as part of the integration test suite without conflicting with the actual AWS based tests.