chore: remove ref from checkout workflow (#27490)

Issue number: N/A

---------

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

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
eb2772c0ce.
However,
381de0b3d3
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?
<!-- Please describe the behavior or changes that are being added by
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

<!-- If this introduces a breaking change, please describe the impact
and migration path for existing applications below. -->


## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

.github/workflows/actions/build-core/action.yml

@@ -4,9 +4,6 @@ runs:
   using: 'composite'
   steps:
     - uses: actions/checkout@v3
-      with:
-        # Checkout the latest commit in this branch
-        ref: ${{ github.event.pull_request.head.sha }}
     - uses: actions/setup-node@v3
       with:
         node-version: 16.x

.github/workflows/build.yml

@@ -17,9 +17,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-        with:
-          # Checkout the latest commit in this branch
-          ref: ${{ github.event.pull_request.head.sha }}
       - uses: ./.github/workflows/actions/build-core
 
   test-core-clean-build: