iosevka: get set value from privateBuildPlan, if it's an attrset

I find it redundant to set `set`, when a config is supplied.

pkgs/by-name/io/iosevka/package.nix

@@ -47,10 +47,12 @@
   #   sequence = "+>"
   # '';
   extraParameters ? null,
-  # Custom font set name. Required if any custom settings above.
-  set ? null,
+  # Custom font set name. Required if `extraParameters` is used and/or
+  # if `privateBuildPlan` is a TOML string.
+  set ? privateBuildPlan.family or null,
 }:
 
+assert (builtins.isAttrs privateBuildPlan) -> builtins.hasAttr "family" privateBuildPlan;
 assert (privateBuildPlan != null) -> set != null;
 assert (extraParameters != null) -> set != null;