Supply a name to cleanSourceWith to suppress build-time warning

trace: WARNING: cleanSourceWith called on /nix/store/l74k6l0sw0dfqnykxxa9zazlbxkqdb8n-source without a name. Consider adding name = "l74k6l0sw0dfqnykxxa9zazlbxkqdb8n-source";

I cargo-culted this from https://input-output-hk.github.io/haskell.nix/tutorials/getting-started.html#scaffolding , hoping it does the right thing.