CI: Cache build folder on Windows

Since the initial `cmake -G "Visual Studio 17 2022" ..` seems to be a
real bottleneck on the Windows CI (sometimes taking multiple minutes
for seemingly no reason), cache the build folder based on the hash
of CMakeLists.txt. It should be possible to reuse the build folder if
CMakeLists hasn't changed, and if it does change (because someone adds
a source file or library or something), we'll just do that initial
cmake with -G once, and cache that version as well.

https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows

.github/workflows/ci.yml

@@ -139,7 +139,17 @@ jobs:
         Invoke-WebRequest "https://github.com/libsdl-org/SDL/releases/download/release-$env:SDL_VERSION/SDL2-devel-$env:SDL_VERSION-VC.zip" -o C:\SDL.zip
         Expand-Archive C:\SDL.zip -DestinationPath C:\
 
-    - name: CMake configure (default version)
+    - name: Cache build folder for this CMakeLists.txt
+      id: cache-windows-build-folder
+      uses: actions/cache@v3
+      env:
+        cache-name: cache-windows-build-folder-VS2022
+      with:
+        path: desktop_version/build
+        key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('desktop_version/CMakeLists.txt') }}-SDL${{ env.SDL_VERSION }}
+
+    - if: ${{ steps.cache-windows-build-folder.outputs.cache-hit != 'true' }}
+      name: CMake initial configure/generate
       run: |
         mkdir $env:SRC_DIR_PATH/build
         cd $env:SRC_DIR_PATH/build
@@ -147,6 +157,11 @@ jobs:
         cmake -G "Visual Studio 17 2022" -A Win32 `
               -DSDL2_INCLUDE_DIRS="C:\SDL2-$env:SDL_VERSION\include" `
               -DSDL2_LIBRARIES="SDL2;SDL2main" ..
+
+    - name: CMake configure (default version)
+      run: |
+        cd $env:SRC_DIR_PATH/build
+        cmake ..
     - name: Build (default version)
       run: |
         cd $env:SRC_DIR_PATH/build