[github] Fix checkout of PR base commit in pr.yml

[kainino0x]

This should make the case-count report work better.

It was checking out the latest version of main instead of the main-branch commit that GitHub picked as the base commit when creating its temporary merge commit to run the action. This can be different (e.g. if the main branch changes while the tests are running, though I'm not sure if that's what actually happened in the run where I encountered this issue). Fix this by only fetching once, and using the exact commit that the merge was based off.

Issue: none

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Requirements for PR author:

• [n/a] All missing test coverage is tracked with "TODO" or .unimplemented().
• [n/a] New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [n/a] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [n/a] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• [n/a] Tests are properly located.
• [n/a] Test descriptions are accurate and complete.
• [n/a] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• [n/a] Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at ba7982f):

 Test case/subcase counts did not change.