This allows setting `$PINNIPED_TEST_CLI` to point at an existing `pinniped` CLI binary instead of having the test build one on-the-fly. This is more efficient when you're running the tests across many clusters as we do in CI.
Building the CLI from scratch in our CI environment takes 1.5-2 minutes, so this change should save nearly that much time on every test job.
Signed-off-by: Matt Moyer <moyerm@vmware.com>
