Fix global config load

This fix prevents an error when `$HOME` is not set in the build environment. (Then `~` can not be expanded)

I'm not sure how to write a unit test for this, so I didn't. Tried to use the branch on environments with and without `$HOME`, which worked as expected.
Actually, I think I spoke too soon, as the '~' as pre-existing.  I do think there should be a more general solution.

```ruby
require 'etc'
home_dir = Dir.home || Etc.getpwuid.dir || (user = ENV["USER"] && Dir.home(user))
```