"ghcup set ghc next" command.

I've previously manually installed GHCs and had a little bash script for switching between GHC versions.

One nice thing was that the command was just g and without any options it would rotate all available GHC versions on my machine. So I would just bang g three times until I was at the GHC I wanted.

It would be nice to have this functionality as part of ghcup. A new next tag on set could make the switch. I could then alias it to g again.