## Summary
Closes elastic/kibana-operations#110

This is a continuation of elastic#217929. Previously a `Task` created two
artifacts of a specific flavor, one for `x64` and one for `ARM64`. These
were still running serially, even though the `Task` itself was
parallelized. This PR splits each architecture into its own `Task` so we
can increase parallelism. The result is a 14% further decrease in time
taken for the build step; 53 min down to 46 min. Between the two PRs,
the artifact build time is reduced 57%!

### Testing
https://buildkite.com/elastic/kibana-artifacts-snapshot/builds/6413

### Next steps
- Moving the ARM artifact builds to an ARM native machine would likely
see the next biggest reduction because they take 2-3 times longer than
their x86 counterparts. Though this would likely be a decent amount of
work.
- Some of the `global` tasks could be grouped into parallel tasks, but
there is some dependency between those tasks and the gains would be
minimal.

(cherry picked from commit b51fab2)