soft-serve: set meta.mainProgram to "soft"

This allows `nix run nixpkgs#soft-serve` to function, as ordinarily it would attempt to run `soft-serve`, which isn't present in the `bin` of the output.

pkgs/servers/soft-serve/default.nix

@@ -27,6 +27,7 @@ buildGoModule rec {
   meta = with lib; {
     description = "A tasty, self-hosted Git server for the command line";
     homepage = "https://github.com/charmbracelet/soft-serve";
+    mainProgram = "soft";
     license = licenses.mit;
     maintainers = with maintainers; [ penguwin ];
   };