ci(wasm-size): compare with the base branch and not self

This was commented out in
#4713 to make the pipeline
pass in the PR because the changes weren't compatible with the pipeline
on `main` with the intention to uncomment immediately after landing the
changes, see
#4713 (comment).

.github/workflows/wasm-size.yml

@@ -50,8 +50,8 @@ jobs:
     steps:
       - name: Checkout base branch
         uses: actions/checkout@v4
-        # with:
-        #   ref: ${{ github.event.pull_request.base.sha }}
+        with:
+          ref: ${{ github.event.pull_request.base.sha }}
 
       - uses: ./.github/workflows/include/rust-wasm-setup