test: Add integration tests for FUNC_USERNAME/FUNC_PASSWORD across all builders

[Kunal1522]

Summary

Add integration tests to verify that FUNC_USERNAME/FUNC_PASSWORD environment variables correctly pass credentials to the pusher for all builders (host, s2i, pack).
These tests will provide the confidence needed to unhide the --username/--password flags and officially support them across all builders.

Background

Credential handling is tested at the pusher level, but there's no integration test that verifies the full flow:

[Kunal1522]

/assign