Add "Set Executables Search Path" option on macOS

[eugenesvk]

The guide tells that there is an option to define custom paths, but only on Windows Would you please add the same option on a Mac? The gocryptfs isn't easy to build there (it's missing from Homebrew due to osxfuse's license), so would be nice if you could set path to a custom folder where e.g. Go builds gocryptfs

(of course, you could move the gocryptfs binary to another location in Path, but it's a bit easier not have to worry about that on rebuilds and just set the path in sirikali once)

[mhogomchungu]

Done and the UI now looks like above on Linux and macOS. In macOS, the default location is "/opt/homebrew/bin".