[coverage] speed up merging results of functional tests

[dmlemeshko]

Summary

Currently we iterate through json files generated in functional tests and replace basePath with repo checkout path. It takes 28-30 min:

15:37:00  36G	/tmp/extracted_coverage/target/kibana-coverage
15:37:00  ### Functional: replacing path in json files
16:05:38  ### Merge coverage reports

This PR copies kibana to /dev/shm/workspace/ to match basePath in json files and avoid updating these files.

Before:

### Merge coverage reports - (52 min in self)

After:

### Merge coverage reports - (23 min in self)

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: a415fe3
• Storybooks Preview

Metrics [docs]

✅ unchanged

History

• 💚 Build #108226 succeeded 21252ae

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[LeeDr]

LGTM - I verified the approximately the same number of docs in the output of this branch vs previous