Update Tipf to key off of env.IsGCP() instead of using stackid == "google" to determine if the build is occuring on GCP.
Update Tipf to key off of env.IsGCP() instead of using stackid == "google" to determine if the build is occuring on GCP.

There are new stacks that can work across all three platforms so this logic is no longer valid.

PiperOrigin-RevId: 441211440
Change-Id: I0bd35fa065b1800cbb999f9c3fd4a800c7b4bdff

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks