ci: use pull request head commit whenever possible

The `github.sha` is a merge commit with the parents of latest master
and the head of the pr. Trying to diff the changes from that merge
SHA to base will show all changes that have been made in-between.

And that doesn't seem about right.

We switch to `github.event.pull_request.head.sha` if there is a pr.

See: https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/version-bump.20CI.20failing

.github/workflows/main.yml

@@ -87,7 +87,7 @@ jobs:
     runs-on: ubuntu-latest
     env:
       BASE_SHA: ${{ github.event.pull_request.base.sha }}
-      HEAD_SHA: ${{ github.sha }}
+      HEAD_SHA: ${{ github.event.pull_request.head.sha != '' && github.event.pull_request.head.sha || github.sha  }}
     steps:
       - uses: actions/checkout@v3
         with: