nom DerivationParseError with impure-derivations experimental feature

[MatthewCroughan]

Looks like impure-derivations look slightly different than normal derivations, and contain an unexpected field.

user: matthew ~/tmp/h via ❄️  impure (shell) 
❯ cat f.nix 
{ stdenv }:
stdenv.mkDerivation {
  name = "impure";
  __impure = true; # marks this derivation as impure
  outputHashAlgo = "sha256";
  outputHashMode = "recursive";
  buildCommand = "date > $out";
}

user: matthew ~/tmp/h via ❄️  impure (shell) 
❯ nom build --impure --expr '(import <nixpkgs> {}).callPackage ./f.nix {}' -L
this derivation will be built:
nix-output-monitor error: DerivationParseError "string"
  /nix/store/ip3aw29yb2fvrd9ia5m7s3zd64lqjpip-impure.drv
resolved derivation: '/nix/store/ip3aw29yb2fvrd9ia5m7s3zd64lqjpip-impure.drv' -> '/nix/store/skhbmasyz1cbb8yn47vfgw6sbwslllxy-impure.drv'
nix-output-monitor error: DerivationParseError "string"
nix-output-monitor error: DerivationParseError "string"
nix-output-monitor error: DerivationParseError "string"
impure> building '/nix/store/skhbmasyz1cbb8yn47vfgw6sbwslllxy-impure.drv'
nix-output-monitor error: DerivationParseError "string"
┏━━━ Builds         
┗━ ∑ ⏵ 1 │ ✔ 0 │ ⏸ 1 │ Finished at 09:46:16 after 1s