Cache the CMake index file during CI

We're currently caching the dependency source and build files, but CMake
is still recompiling the dependency build files since it doesn't know where they
are.

PiperOrigin-RevId: 696626931

Author: dhoekwater

.github/workflows/ci.yaml

@@ -67,6 +67,7 @@ jobs:
             build/_deps/*-build
             build/_deps/*-subbuild
             build/*-build
+            build/CMakeFiles/TargetDirectories.txt
           key: ${{ runner.os }}-cmake-build-${{ hashFiles('**/CMakeLists.txt', 'CMake/**') }}
           restore-keys: |
             ${{ runner.os }}-cmake-build-