Thanks, this was a big help! I got the cannot load /sonic-pi/app/server/ruby/rb-native/2.5.0/rugged error reported in the OP today, when trying to build Sonic Pi 3.2.0 on Ubuntu 18.10. After following your instructions for downloading the gem and copying it to the correct folder, I was able to run the build-ubuntu-18-04 script without any errors.
I didn’t update the compile-extensions.rb file to the correct version of rugged. The version of rugged that I copied to the vendor directory was rugged-0.28.1. Looking at the vendor directory now, I see that both rugged-0.28.1 and rugged-0.26.0 (the version that’s referenced in compile-extensions.rb ) are present. I’m not sure what is going on with that.
Sonic Pi is working great for me now. Unfortunately, I didn’t pay as much attention to the build process as I could have. I’d be happy to go through the process again if it could help to debug the Ubuntu build script or update https://github.com/samaaron/sonic-pi/blob/master/INSTALL-LINUX.md.