Integration tests use distinct label per test run to avoid aborted test causing failures

The GCE instances spawned during the integration tests are using static labels, when a test run aborts, it could fail the subsequent test execution.

Instead use distinct label per test run. (such as label value based on a timestamp maybe)

Check how it's done in `kubernetes` plugin https://github.com/jenkinsci/google-compute-engine-plugin/pull/530#discussion_r2984221379