Support running with unwritable homedir

[Krinkle]

This follows-up from #3 and #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 for code coverage), fail to operate in CI as it tries to unconditionally make a (temporary) directory in HOME.

Downstream:

• spawn-wrap causes failure due to unwritable homedir nyc#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.

[demurgos]

An alternative would be to use /tmp. spawn-wrap doesn't really have to use the home directory, any directory with write access should work. XDG_CACHE_HOME is also a fitting location to use.

[Krinkle]

@demurgos This library originally used /tmp, but this was changed due to #3.

It needs both write access and allow executable files.