Experiment with managing Go and golangci-lint with Nix

[jbduncan]

https://nix.dev/tutorials/first-steps/

[jbduncan]

This would let me replace eg, which is a bit slow, with ast-grep.

[jbduncan]

ast-grep can't check the types of variables and arguments, which eg can do, so I'll probably need both tools eventually as they complement each other.

Also, I've managed to get all my eg templates running in parallel via x.sh, so this is less of a problem.