Add ccache support for faster incremental builds (#301)

## Description of change
Adds `ccache` support for local compiling and Github Actions
environments. This significantly speeds up incremental builds, but cold
starts will take a little longer to compile.

The build script now has an `-i` flag to ignore the build cache if
needed. You can also manually delete the `.ccache` folder to force a
fresh build too.

For the GA workflow, it uses the
[`ccache-action`](https://github.com/hendrikmuhs/ccache-action) action
to manage and store the cached build objects. This cache should be
accessible to all branches.

I also removed the `--no-cache` flag when building the
`spicetools/spice` image since Docker will automatically invalidate
`COPY` layers if it detects the copied files have changed.

### Compile times
Compiled with a 7800X3D, 32GB 6000MT/s RAM.
| `main` | `ccache` (fresh) | `ccache` (cached) | `ccache` (ignore
cache) |
|--------|--------|--------|--------|
| 2m46.047s | 3m8.886s | 0m32.638s | 2m45.148s |

## Testing
Tested incremental builds by using another branch that's rebased on top
of this.

src/spice2x/build_docker.bat

@@ -1,6 +1,6 @@
 docker build --pull external/docker -t spicetools/deps
-docker build --build-context gitroot=%cd%/../../.git . -t spicetools/spice --no-cache
-docker run --rm -it -v %cd%/dist:/src/src/spice2x/dist -v %cd%/bin:/src/src/spice2x/bin spicetools/spice
+docker build --build-context gitroot=%cd%/../../.git . -t spicetools/spice
+docker run --rm -it -v %cd%/dist:/src/src/spice2x/dist -v %cd%/bin:/src/src/spice2x/bin -v %cd%/.ccache:/src/src/spice2x/.ccache spicetools/spice %*
 @REM to generate PDBs, set DEBUG to 1 in build_all.sh, place cv2pdb in external\cv2pdb, and run below
 @REM external\cv2pdb\cv2pdb.exe bin\spice2x\spicecfg.exe bin\spice2x\spicecfg-pdb.exe bin\spice2x\spicecfg-pdb.pdb
 @REM external\cv2pdb\cv2pdb.exe bin\spice2x\spice.exe bin\spice2x\spice-pdb.exe bin\spice2x\spice-pdb.pdb