Build cache (ccache) path currently doesn't work properly in CI

[fwcd]

$HOME is not resolved in this context, so the current solution does not work:

https://github.com/fwcd/m1xxx/blob/fa780e533241898be48a2bbdca9f3d704e867867/.github/workflows/build.yml#L124

We can, however, use ~ to resolve the directory, see:

• https://stackoverflow.com/questions/65801828/how-do-i-cache-an-installed-tool-between-runs-in-github-actions

48 addresses this by adding ccache_path to the matrix with the correct paths. If that PR turns out to be a larger blocker, we might consider extracting that logic (including the step that saves the ccache even if the run fails).