[ci] investigate moving airgapped build test to GCP runner (instead of azure runner)

### Description

The airgapped build test job has run out of space several times on both integrated (see https://github.com/lowRISC/opentitan-integrated/pull/321) and discrete branches. We should investigate how to move this job to a GCP runner. IIRC, it was not as simple as just updating the CI YAML file, as there were issues with instantiating the network namespace it uses in the containerized environment on the GCP runners (but further investigation is required).

Start with the CI YAML for the airgapped build test and inspect how its containerized environment creates the network namespace on GCP runners. Compare the existing Azure runner configuration with GCP runner constraints, then verify that the job runs successfully without exhausting disk space and that the airgapped networking behavior is preserved.

Written by the indexing model from the issue text.