Don't use 'flutter upgrade' on Cirrus

[stuartmorgan]

This command isn't intended for CI use, and is also slower due to
downloading artifacts that will be immidately discarded.

Cirrus portion of flutter/flutter#86037

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[jmagman]

@christopherfujino This looks right to me, can you think of any more gotchas to resetting since you're been conductoring?

[christopherfujino]

LGTM. The only difference under the hood between these commands and doing flutter upgrade is that the latter will also precache certain artifacts. However, the tool should ensure any necessary cache artifacts in the validation phase of any command (e.g. ios artifacts before flutter build ios), so this should be fine.

[jmagman]

LGTM