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.

.github/workflows/ci.yml

@@ -13,6 +13,11 @@ jobs:
         working-directory: ./src/spice2x
     steps:
       - uses: actions/checkout@v4
+      - name: Set ccache environment variables
+        run: |
+          echo "CCACHE_DIR=${{ github.workspace }}/src/spice2x/.ccache" >> $GITHUB_ENV
+      - name: Install ccache
+        uses: hendrikmuhs/ccache-action@v1.2
       - name: Calculate commit SHA
         id: vars
         run: |