ci: switch all caches to actions/cache (drop WarpBuilds/cache)

WarpBuild's cache service is billed; GitHub's cache is free (10 GB
repo limit with LRU eviction, ample for ccache entries). All three
cache sites — ccache for the matrix and dynamic-backend jobs, plus the
macOS OpenTimelineIO build — now use actions/cache@v4, which also
removes the Windows-specific fallback step.

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
       - name: Cache OpenTimelineIO (macOS)
         if: runner.os == 'macOS'
         id: otio-cache
-        uses: WarpBuilds/cache@v1
+        uses: actions/cache@v4
         with:
           path: otio-install
           key: otio-0.16.0-${{ runner.os }}-v1
@@ -108,19 +108,6 @@ jobs:
       # flags below pick it up on all three platforms
       # ------------------------------------------------------------------
       - name: Restore ccache
-        if: runner.os != 'Windows'
-        uses: WarpBuilds/cache@v1
-        with:
-          path: ${{ github.workspace }}/.ccache
-          key: ccache-${{ runner.os }}-${{ github.ref_name }}-${{ github.sha }}
-          restore-keys: |
-            ccache-${{ runner.os }}-${{ github.ref_name }}-
-            ccache-${{ runner.os }}-
-
-      # WarpBuilds/cache does not support Windows runners; use GitHub's
-      # cache service there (separate backend, same keys)
-      - name: Restore ccache (Windows)
-        if: runner.os == 'Windows'
         uses: actions/cache@v4
         with:
           path: ${{ github.workspace }}/.ccache
@@ -308,7 +295,7 @@ jobs:
             portaudio19-dev libgl1-mesa-dev libxkbcommon-dev ccache
 
       - name: Restore ccache
-        uses: WarpBuilds/cache@v1
+        uses: actions/cache@v4
         with:
           path: ${{ github.workspace }}/.ccache
           key: ccache-${{ runner.os }}-dynamic-${{ github.ref_name }}-${{ github.sha }}