api: skip retired-env gate test when the var is present-but-empty

The unset-passes subtest guarded with Getenv != "" while the gate rejects a present-but-empty ENTIRE_AUTH_BASE_URL too, so an empty export in the parent shell ran the subtest and failed it. LookupEnv matches what the gate checks.

Deliberately NOT scrubbing the var in the integration/e2e TestMains: a shell that still exports it fails every entire invocation loudly by design, and the test harness shouldn't mask that.