chore: remove ref from checkout workflow (#27490)

Issue number: N/A

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## What is the current behavior?
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Screenshots in a branch are currently compared against the screenshots
on `main`. While we ultimately need to compare against ground truths in
`main`, this can cause tests to seemingly fail at random. For example,
this branch was created based on
https://github.com/ionic-team/ionic-framework/commit/eb2772c0ce623de70cce42b5d36e86cdbba7bafb.
However,
https://github.com/ionic-team/ionic-framework/commit/381de0b3d324805161232d8556fffd7022fcd84c
was added to `main` after the first commit and has new screenshot diffs.

I do not have the latest ground truths for the changed screenshots in
this branch, so the screenshot tests failed in
https://github.com/ionic-team/ionic-framework/actions/runs/4995702351/jobs/8947969061
even though I did not change any component implementations.

## What is the new behavior?
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this PR. -->

- Test runs in CI now run against the ground truths available in the
branch. When a commit is added to the merge queue, it will be tested
against the latest `main` and any commits ahead of it in the queue. As a
result, if there _is_ a conflict then the test run will fail at that
point and the commit will be removed from the merge queue.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->