use builtins.fromYAML

[cdepillabout]

There is currently a PR to add a fromYAML builtin to Nix:

NixOS/nix#7340

stacklock2nix should switch to using this if it gets merged. This will remove one use of IFD (for reading the stack.yaml and stack.yaml.lock files).

Note that we can't out-right remove the fromYAML function defined in stacklock2nix, because we still need to support old versions of Nix that don't yet have the fromYAML builtin.