Look for Sonic Pi server in ~/Applications

[yaychris]

I have Sonic Pi installed in ~/Applications, a standard place to install applications on macOS. sonic-pi-tool doesn't look there, so I'm not able to use sonic-pi-tool start-server.

This function would be updated to implement this request, using std::env::home_dir().

I attempted to do so, but was unable to make it work because I don't know Rust well enough to appease the sorrow checker.