ci: try conditional instead

Author: danielroe

.github/workflows/size.yml

@@ -1,6 +1,9 @@
 name: size
 on:
   # this action will error unless run in a pr context
+  pull_request_target:
+    branches:
+      - main
   pull_request:
     branches:
       - main
@@ -18,8 +21,14 @@ jobs:
       contents: read
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        if: github.event_name == 'pull_request'
         with:
           ref: ${{ github.event.pull_request.head.sha }}
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+        if: github.event_name == 'pull_request_target'
+        with:
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
       - run: npm i -g --force corepack && corepack enable
       - uses: actions/setup-node@1d0ff469b7ec7b3cb9d8673fde0c81c44821de2a # v4.2.0
         with: