ci: fix service variable visibility

We need to explicitly set GCP_OIDC_* to be available in service
container as they can't directly access pipeline variables [1].

We also need to use intermediate variable names as substitution does
not seem to work if we try to set a variable with a reference to a
variable with the same name. Could not find good reference about the
issue though.

Fixes test-token-succeeds job failure

[1] https://docs.gitlab.com/ee/ci/variables/index.html#in-service-containers