Support running with unwritable homedir

This follows-up from https://github.com/tapjs/spawn-wrap/issues/3 and https://github.com/tapjs/spawn-wrap/issues/49.

The CI environment I use for some projects runs builds in a Docker container as `nobody` and its default of `HOME=/nonexistent`. Due to this, projects using spawn-wrap (such as [istanbuljs/nyc](https://github.com/istanbuljs/nyc/issues/951) for code coverage), fail to operate in CI as it tries to unconditionally make a (temporary) directory in HOME.

Downstream:
* https://github.com/istanbuljs/nyc/issues/951
* https://phabricator.wikimedia.org/T212602#4847719

Would you be open toward a patch that considers `XDG_CACHE_HOME` before falling back to `HOME`?

This way, we wouldn't have to maintain an hardcoded exception in our CI environment (or in `package.json#scripts.test`) that sets `SPAWN_WRAP_SHIM_ROOT`. I imagine this would be beneficial to other users as well.

Thanks.