ci: move the three Linux jobs to WarpBuild's cache

The GitHub Actions cache quota is full (the vcpkg trees plus the cargo
target dirs overflow the 10 GiB repo budget). The Linux job and both
openKylin matrix jobs now use WarpBuild's drop-in cache actions
(WarpBuilds/cache restore+save, WarpBuilds/rust-cache); all of them run
on WarpBuild runners, where the service is available. macOS and Windows
keep actions/cache for now.

.github/workflows/ci.yml

@@ -74,9 +74,11 @@ jobs:
       # rebuilds only what changed. Every run saves under a fresh key
       # (so an existing cache is updated, never a save failure) and
       # restores the newest matching entry through `restore-keys`.
+      # WarpBuild's cache service backs this job (the GitHub Actions
+      # cache quota is full); macOS/Windows keep actions/cache.
       - name: Restore vcpkg artifacts
         id: vcpkg-cache
-        uses: actions/cache/restore@v6
+        uses: WarpBuilds/cache/restore@v2
         with:
           path: |
             vcpkg_installed
@@ -96,7 +98,7 @@ jobs:
       # (~40 min on Windows).
       - name: Save vcpkg artifacts
         if: always()
-        uses: actions/cache/save@v6
+        uses: WarpBuilds/cache/save@v2
         with:
           path: |
             vcpkg_installed
@@ -129,7 +131,7 @@ jobs:
       # Covers the whole target/ dir plus ~/.cargo; shared across branches
       # of the same OS.
       - name: Cache cargo artifacts
-        uses: Swatinem/rust-cache@v2
+        uses: WarpBuilds/rust-cache@v2
         with:
           shared-key: oak-ci-linux
           cache-on-failure: true
@@ -606,9 +608,11 @@ jobs:
 
       # The archives dir is vcpkg's binary cache: a manifest bump then
       # rebuilds only what changed.
+      # WarpBuild's cache service backs this job (the GitHub Actions
+      # cache quota is full); macOS/Windows keep actions/cache.
       - name: Restore vcpkg artifacts
         id: vcpkg-cache
-        uses: actions/cache/restore@v6
+        uses: WarpBuilds/cache/restore@v2
         with:
           path: |
             vcpkg_installed
@@ -625,7 +629,7 @@ jobs:
 
       - name: Save vcpkg artifacts
         if: always()
-        uses: actions/cache/save@v6
+        uses: WarpBuilds/cache/save@v2
         with:
           path: |
             vcpkg_installed
@@ -651,7 +655,7 @@ jobs:
 
       # Covers the whole target/ dir plus ~/.cargo.
       - name: Cache cargo artifacts
-        uses: Swatinem/rust-cache@v2
+        uses: WarpBuilds/rust-cache@v2
         with:
           shared-key: oak-ci-openkylin-${{ matrix.arch }}
           cache-on-failure: true