lib/generators.toPretty: don't evaluate derivations

With the goal of making `toPretty` suitable for rendering option
values, render derivations as `<derivation foo-1.0>` instead of
`<derivation /nix/store/…-foo-1.0.drv>`.

This is to avoid causing sudden evaluation errors for out-of-tree
projects that have options with `default = pkgs.someUnfreePackage;` and
no `defaultText`.

lib/generators.nix

@@ -330,7 +330,7 @@ rec {
          then v.__pretty v.val
       else if v == {} then "{ }"
       else if v ? type && v.type == "derivation" then
-        "<derivation ${v.drvPath or "???"}>"
+        "<derivation ${v.name or "???"}>"
       else "{" + introSpace
           + libStr.concatStringsSep introSpace (libAttr.mapAttrsToList
               (name: value:

lib/tests/misc.nix

@@ -754,7 +754,7 @@ runTests {
       emptylist = "[ ]";
       attrs = "{ foo = null; \"foo bar\" = \"baz\"; }";
       emptyattrs = "{ }";
-      drv = "<derivation ${deriv.drvPath}>";
+      drv = "<derivation ${deriv.name}>";
     };
   };