[no ci] Update pr.yml to cache builds

Signed-off-by: June Park <me@pythonplayer123.dev>

.github/workflows/pr.yml

@@ -39,6 +39,12 @@ jobs:
         with:
           xcode-version: ${{ matrix.version }}
 
+      - name: Cache Build
+        uses: irgaly/xcode-cache@v1
+        with:
+          key: xcode-cache-deriveddata-${{ github.sha }}
+          restore-keys: xcode-cache-deriveddata-
+
       - name: Build SideStore
         run: make build | xcpretty && exit ${PIPESTATUS[0]}