CI: Run [build_octez_source] on [gcp_very_high] cpu runner tag

The CI job build_octez_source reach more and more often OOM.

In this MR, we will try to measure the duration of the job on the gcp_very_high_cpu tag and note the performance gains but also take into account the associated GCP costs.

Result:

• on gcp tag, the median duration is 24.4 minutes.
• on gcp_very_high_cpu, the duration is 5 minutes 5 minutes.