[test] split size report jobs (#11918)

This commit split the size report workflow into two jobs of different
permissions.

.github/workflows/size.yml

@@ -42,9 +42,7 @@ concurrency:
 
 jobs:
 
-  size-report:
-    permissions:
-      pull-requests: write
+  size-check:
     runs-on: ubuntu-24.04
     steps:
     - name: Harden Runner
@@ -65,9 +63,29 @@ jobs:
       run: |
         ./script/check-size
         cat /tmp/ot-size-report/report_pr >> $GITHUB_STEP_SUMMARY
-    - uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea
-      name: Post Report
-      if: ${{ github.event_name == 'pull_request_target' }}
+    - name: Upload report
+      uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+      with:
+        name: report_pr
+        path: /tmp/ot-size-report/report_pr
+
+
+  size-report:
+    needs:
+    - size-check
+    permissions:
+      pull-requests: write
+    if: github.event_name == 'pull_request_target'
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Download report
+      uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4.3.0
+      with:
+        name: report_pr
+        path: /tmp/ot-size-report
+
+    - name: Post Report
+      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea
       id: post-report
       with:
         script: |