Cache the Android NDK in the cmake-android-* workflows.

Currently, `ccache` is not getting any hits, likely because the `mtime` of the compiler binaries changes every time `sttld/setup-ndk` is called (see discussion [here](hendrikmuhs/ccache-action#94)).
With this change, the NDK itself is cached, which hopefully maintains the `mtime` across builds.

PiperOrigin-RevId: 662464572

.github/workflows/build.yml

@@ -282,6 +282,10 @@ jobs:
           key: ${{ github.job }}-${{ matrix.arch }}
           max-size: "1G"
           save: ${{ inputs.update-caches }}
+      - name: Print binary mtimes
+        run: |
+          ls -l /opt/hostedtoolcache/ndk/r23b/x64/toolchains/llvm/prebuilt/linux-x86_64/bin/clang*
+          file /opt/hostedtoolcache/ndk/r23b/x64/toolchains/llvm/prebuilt/linux-x86_64/bin/clang*
       - name: Configure and build
         run: scripts/build-android-${{ matrix.arch }}.sh
         working-directory: ${{ github.workspace }}