(Also I got permission denied when attempting to curl the script when writing to /usr/local/bin, I needed sudo. I'm on Ubuntu 19.10 Eoan Ermine. Not sure whether security implications for `curl | sh` outweigh convenience, but I trust you guys and my connection. :P)
The script is just picking the binary for your OS/arch and putting in PATH. We have instructions for doing it yourself here https://fly.io/docs/getting-started/installing-flyctl/#comma...
Or you can download straight from github: https://github.com/superfly/flyctl/releases
Hopefully we can get on snap soon!