CI/pr-size: Use pull_request_target to get write permission

- Build and use pr-size from the base branch
- Compare against the PR in a second step

.github/workflows/pr-size.yml

@@ -1,7 +1,7 @@
 name: pr-size
 
 on:
-  pull_request:
+  pull_request_target:
     branches: [ MK3, MK3_* ]
 
 env:
@@ -10,6 +10,8 @@ env:
 jobs:
   build:
     runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
 
     steps:
 
@@ -18,34 +20,14 @@ jobs:
       run: |
           sudo apt-get install cmake ninja-build python3-pyelftools python3-regex python3-polib
 
-    # build the PR branch
-    - name: Checkout PR
+    # build the base branch
+    - name: Checkout base
       uses: actions/checkout@v3
 
     - name: Setup build dependencies
       run: |
           ./utils/bootstrap.py
 
-    - name: Build PR
-      run: |
-          rm -rf build-pr
-          mkdir build-pr
-          cd build-pr
-          cmake .. -DCMAKE_TOOLCHAIN_FILE="../cmake/AvrGcc.cmake" -DCMAKE_BUILD_TYPE=Release -G Ninja
-          ninja $TARGETS
-
-    # save pr-size for later use
-    - name: Save pr-size from PR
-      run: |
-          cp -f ./.github/workflows/pr-size.sh build-pr
-
-    # build the base branch
-    - name: Checkout base
-      uses: actions/checkout@v3
-      with:
-        clean: false
-        ref: ${{ github.event.pull_request.base.ref }}
-
     - name: Build base
       run: |
           rm -rf build-base
@@ -54,11 +36,31 @@ jobs:
           cmake .. -DCMAKE_TOOLCHAIN_FILE="../cmake/AvrGcc.cmake" -DCMAKE_BUILD_TYPE=Release -G Ninja
           ninja $TARGETS
 
+    # save pr-size for later use
+    - name: Save base data
+      run: |
+          cp -f ./.github/workflows/pr-size.sh build-base
+
+    # build the PR branch
+    - name: Checkout PR
+      uses: actions/checkout@v3
+      with:
+        clean: false
+        ref: ${{ github.event.pull_request.head.sha }}
+
+    - name: Build PR
+      run: |
+          rm -rf build-pr
+          mkdir build-pr
+          cd build-pr
+          cmake .. -DCMAKE_TOOLCHAIN_FILE="../cmake/AvrGcc.cmake" -DCMAKE_BUILD_TYPE=Release -G Ninja
+          ninja $TARGETS
+
     # extract/show build differences
     - name: Calculate binary changes
       run: |
           rm -rf build-changes
-          ./build-pr/pr-size.sh build-changes build-base build-pr $TARGETS
+          ./build-base/pr-size.sh build-changes build-base build-pr $TARGETS
 
     - name: Add PR Comment
       uses: mshick/add-pr-comment@v2