Add PIPSI_HOME/PIPSI_BIN_DIR support to installer
Add PIPSI_HOME/PIPSI_BIN_DIR support to installer

Expands support for the `PIPSI_HOME` and `PIPSI_BIN_DIR` environment
variables introduced in 09fd44a.

If set, the installer will attempt to install Pipsi to
`$PIPSI_HOME/pipsi` and symlink the executable at
`$PIPSI_BIN_DIR/pipsi`.

blueyed/pipsi,RaviTezu/pipsi,habnabit/pipsi