Save/Restore seperately in MacOS/Windows caching workflows. (#11199)

Only *save* cache on the default branch (develop), restore it everywhere it fits

.github/workflows/build_windows_bin.yml

@@ -14,6 +14,10 @@ permissions:
 jobs:
   build-Windows:
     runs-on: windows-${{ inputs.windows_ver }}
+    # Only pushes to the default branch (develop) populate the cache; PR / merge_group runs
+    # restore it but never save, so they stop filling up the repo's Actions cache storage.
+    env:
+      SAVE_CACHE: ${{ github.event_name == 'push' && github.ref_name == github.event.repository.default_branch }}
     defaults:
       run:
         # UCRT64 is the MinGW-w64 environment native-windows targets; the `msys`
@@ -71,8 +75,9 @@ jobs:
           python -m pip install --upgrade pip
           pip install platformio
 
-      - name: Cache PlatformIO packages
-        uses: actions/cache@v6
+      - name: Restore PlatformIO cache
+        id: pio-cache
+        uses: actions/cache/restore@v6
         with:
           path: ~/.platformio/.cache
           key: pio-windows-${{ inputs.windows_ver }}-${{ hashFiles('platformio.ini', 'variants/native/portduino.ini') }}
@@ -94,6 +99,13 @@ jobs:
         env:
           PKG_VERSION: ${{ steps.version.outputs.long }}
 
+      - name: Save PlatformIO cache
+        if: env.SAVE_CACHE == 'true' && steps.pio-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v6
+        with:
+          path: ~/.platformio/.cache
+          key: pio-windows-${{ inputs.windows_ver }}-${{ hashFiles('platformio.ini', 'variants/native/portduino.ini') }}
+
       - name: List output files
         run: ls -lah .pio/build/native-windows/