Resolve the TODO at test/e2e/util/options.go line 454: "TODO: make this an envvar ... then change here".

Why

The E2E test base domain is currently baked into the code as a default value. CI jobs and developers running tests in different environments cannot easily override this without modifying source code.

Acceptance Criteria

• Introduce a BASE_DOMAIN (or similar) environment variable
• Wire it into ConfigurableClusterOptions.BaseDomain
• Fall back to the existing default if the env var is not set
• Update any CI documentation if applicable
• Remove the TODO comment from the source

Technical Details

• File: test/e2e/util/options.go line 454
• TODO text: "TODO: make this an envvar ... then change here"
• Context: ConfigurableClusterOptions.BaseDomain default value