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.

(cherry picked from commit 4910d74086a85876e093136a0e8ebc547b467af7)

Change-Id: I8e1cb39c05db812080759ec183ee7a131760e6ea

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
+                    }
+                )
+            );
         }
     }
 };