[ci] Pin master in Cirrus

[stuartmorgan-g]

This uses a pinned version from a file instead of master when checking out Flutter for master channel testing in Cirrus. Currently it's a static version, but this will be updated by an auto-roller.

This uses a version of master from several days ago to avoid the current tree breakage due to flutter/flutter#96661

This will not affect LUCI tests; that will require a separate recipe change.

Part of flutter/flutter#93811

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. (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, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[gaaclarke]

Do we want to wait until we have an autoroller before we land this?

[stuartmorgan-g]

Well, we need the text file here for the autoroller to be set up. Normally I would have done it as:

1. Just add the text file
2. Get the autoroller set up and working
3. Make the other changes here

But right now the tree has been closed for a day already, and re-opening it requires a fix-forward in the engine that hasn't even been written yet. It needs to be written, reviewed, landed, and rolled into the framework before master will work for us again. Given that, landing everything with a static pinned version is strictly better, as it means we can re-open the tree now instead.

If it turns out that the autoroller takes longer to set up than master being fixed, then I can just temporarily revert part of the .cirrus.yaml file once master works.

[gaaclarke]

lgtm

[stuartmorgan-g]

Landing on red to re-open tree.