ci: check out PR head commits in workflows (#21835)

## Why

PR CI should test the exact commit that was pushed to the PR branch. By
default, GitHub's `pull_request` event checks out a synthetic merge
commit from `refs/pull/<number>/merge`, so the tested tree can include
an implicit merge with the current base branch instead of matching the
pushed head SHA.

Using the PR head SHA makes each check result correspond to a concrete
commit the author submitted. This also behaves better for stacked PR
workflows, including Sapling stacks and other Git stack tooling: a
middle PR's head commit already contains the lower stack changes in its
tree, without pulling in commits above it or GitHub's temporary merge
ref.

## What Changed

- Set every `actions/checkout` in `pull_request` workflows under
`.github/workflows` to use `github.event.pull_request.head.sha` on PR
events and `github.sha` otherwise.
- Updated `blob-size-policy` to compare
`github.event.pull_request.base.sha` and
`github.event.pull_request.head.sha`, since it no longer checks out
GitHub's merge commit where `HEAD^1`/`HEAD^2` represented the PR range.

## Verification

- Parsed the edited workflow YAML files with Ruby.
- Checked that every checkout block in the `pull_request` workflows has
the PR-head `ref`.

.github/workflows/codespell.yml

@@ -20,6 +20,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
       - name: Annotate locations with typos
         uses: codespell-project/codespell-problem-matcher@b80729f885d32f78a716c2f107b4db1025001c42 # v1.1.0