build-size-workflow: Fix when base gets updated

The github.event.pull_request.base.sha in the workflow doesn't get
updated when there are new commits in the base branch. Instead always
checkout the branch to check the sha manually and cache only the build.

The ownership fix seems to still be necessary

.github/workflows/main.yml

@@ -113,21 +113,27 @@ jobs:
       # Override SOURCES_DIR in build.sh
       SOURCES_DIR: .
     steps:
-    - name: Cache sources
-      id: cache-sources
-      uses: actions/cache@v3
-      with:
-        path: .
-        key: source-files-${{ github.event.pull_request.base.sha }}
-
-    - if: ${{ steps.cache-sources.outputs.cache-hit != 'true' }}
-      name: Checkout source files
+    - name: Checkout current base branch files
       uses: actions/checkout@v3
       with:
         ref: ${{ github.base_ref }}
         submodules: recursive
 
-    - if: ${{ steps.cache-sources.outputs.cache-hit != 'true' }}
+    - name: Get base branch SHA
+      id: get-base-sha
+      run: |
+        # Fix for "detected dubious ownership in repository at '/__w/InfiniTime/InfiniTime'"
+        git config --global --add safe.directory /__w/InfiniTime/InfiniTime
+        echo base_sha=$(git rev-parse HEAD) >> $GITHUB_OUTPUT
+
+    - name: Cache build
+      id: cache-build
+      uses: actions/cache@v3
+      with:
+        path: ./build
+        key: build-files-${{ steps.get-base-sha.outputs.base_sha }}
+
+    - if: ${{ steps.cache-build.outputs.cache-hit != 'true' }}
       name: Build
       shell: bash
       # Only pinetime-app target is needed, but post_build.sh fails