Proper method for preventing default inheritance

[From Matrix](https://matrix.to/#/!gNlHetNvseiGJNBrFZ:matrix.org/$ue6ETXoNXqnK7sjEBHNfbHN9D3g0Q_wgmowZuSvJoik?via=matrix.org):

@nuwang
> There's no way currently to stop it from inherting the default I believe. So you could try setting env: [] explicitly on _conda_local, but that may or may not work

@natefoo
> Yeah, things are working as I have them now (by making the upload and data source tools have their own rule on the default tool) but it does seem like some way to prevent the default inheritance would be ideal.

@nuwang
> Looking at the code, setting inherits to an empty string may work
> But yeah, we should probably add that if not

I can confirm that setting `inherits` to an empty string does indeed work, but as discussed, a proper method (YAML `null`?) would be preferable.