Make global bin dir configurable

[sebastianheuer]

#105 changed the dynamic determination of the target directory for global installations to fixed directories for Linux (/usr/bin) and macOS (/usr/local/bin). It should be possible to override this, preferably by executing a configuration command or manually editing Phive's global configuration file (which will be introduced with #62).

[sgohl]

I am in need of this feature too. It would be convenient to be able to set this via environment variable. This way, it would be perfectly usable in Docker Images.