feat(config): support `$(location)` for data lookup

Due to executor implementation details, we need to use
`$(rlocationpath)` to look up data files when setting env vars in
`labgrid_config`. This is unintuive, and it's likely that users expect
to be able to use `$(location)`, like they would on a `py_binary`, for
example.

We could potentially also support `$(rootpath)` and `$(execpath)`, but
we probably need to understand the use cases a bit better, so I've left
it for later.