Relates to #8104

What

Change the runner used in oc.build_kernels from gcp to gcp_very_high_cpu. (Similar to !20200 (merged).)

Why

oc.build_kernels is often in the critical path, so we should decrease the wall-time of merge_train and before_merging pipelines.
In our tests, we reduce the script_step duration by two. Which means that the cost should be similar.

make -C ci check should pass
look at this timeseries in datadog
- with the gcp runner, the script step duration of oc.build_kernels is either around 2.5 min or around the double of that
- we had one run wich gcp_very_high_cpu and it was about 1.25 min

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR