Print derivation paths in nix eval

`nix eval` forces values and prints derivations as attribute sets, so
commands that print derivations (e.g. `nix eval nixpkgs#bash`) will
infinitely loop and segfault.

Printing derivations as `.drv` paths makes `nix eval` complete as
expected. Further work is needed, but this is better than a segfault.

src/nix/eval.cc

@@ -120,8 +120,17 @@ struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
  }
 
  else {
- state->forceValueDeep(*v);
- logger->cout("%s", ValuePrinter(*state, *v, PrintOptions { .force = true }));
+ logger->cout(
+ "%s",
+ ValuePrinter(
+ *state,
+ *v,
+ PrintOptions {
+ .force = true,
+ .derivationPaths = true
+ }
+ )
+ );
  }
  }
 };