[ci] Cirrus pre-alignment with flutter/plugins, part 1

[stuartmorgan]

Updates .cirrus.yaml to better match flutter/plugins, as a step toward eventually merging the repsositories. Changes include:

• Separating out light workload and heavy workload Linux tasks with different machine specs.
• Enabling analyze on stable in addition to master, which was missing only as an oversight.
• Skips platform tests on stable, as we now do in the plugins repo.
• Separates format and other repo-level, channel-agnostic checks from analyze (mostly to support the above).
• Combines some very cheap steps that don't need to be separate, to reduce machine/setup overhead.
• Cleans up some desktop enabling steps that are no longer necessary.
• Minor flag and comment adjustment to match across repos to aid with diffing.

Since analysis now runs on stable, one analysis option that's not on stable yet had to be commented out.