ci: try head.sha for size ci instead

Author: danielroe

.github/workflows/size.yml

@@ -1,7 +1,6 @@
 name: size
 on:
   # this action will error unless run in a pr context
-  pull_request_target:
   pull_request:
     branches:
       - main
@@ -20,7 +19,7 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
         with:
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.event.pull_request.head.sha }}
       - run: npm i -g --force corepack && corepack enable
       - uses: actions/setup-node@1d0ff469b7ec7b3cb9d8673fde0c81c44821de2a # v4.2.0
         with: