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/external/docker/Dockerfile

@@ -11,6 +11,7 @@ RUN pacman --noconfirm -Syu git \
     mingw-w64-winpthreads \
     mingw-w64-gcc \
     mingw-w64-headers \
-    mingw-w64-binutils
+    mingw-w64-binutils \
+    ccache
 RUN useradd user -m && echo "ALL ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
 RUN su user -c "cd /tmp/ && git clone https://aur.archlinux.org/yay-bin.git --depth=1 && cd yay-bin && makepkg --noconfirm -si && yay --noconfirm -S mingw-w64-cmake"