location input confusing

[lazka]

location is currently used as the parent dir of the installation, so the files will end up in <location>/msys64. It's not clear to users that this is the case.

This comes from the installer/archives containing a toplevel msys64 dir which is extracted. This might make sense for the archives, but for setup-msys2 this is more of an implementation detail.

1. We could document the current location better
2. We could add a new input that specifies the real installation path and deprecate the old one. Not sure how easy that is with the sfx archive though.

Motivated by someone in chat being confused by this.