CI(check-build-tools-image): fix getting build-tools image tag (#7402)

## Problem

For PRs, by default, we check out a phantom merge commit (merge a branch
into the main), but using a real branches head when finding `build-tools`
image tag.

## Summary of changes
- Change `COMMIT_SHA` to use `${{ github.sha }}` instead of `${{
github.event.pull_request.head.sha }}` for PRs

## Checklist before requesting a review

- [x] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

.github/workflows/check-build-tools-image.yml

@@ -28,7 +28,9 @@ jobs:
       - name: Get build-tools image tag for the current commit
         id: get-build-tools-tag
         env:
-          COMMIT_SHA: ${{ github.event.pull_request.head.sha || github.sha }}
+          # Usually, for COMMIT_SHA, we use `github.event.pull_request.head.sha || github.sha`, but here, even for PRs,
+          # we want to use `github.sha` i.e. point to a phantom merge commit to determine the image tag correctly.
+          COMMIT_SHA: ${{ github.sha }}
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           LAST_BUILD_TOOLS_SHA=$(