fix tar command

overlays/cfg-if-fix.nix

···
 final: prev: {
   buildPackage = args:
-    let
-      pname = args.pname or "";
-    in
-    prev.buildPackage (args // (
-      if pname == "cfg-if" then {
-        unpackPhase = ''
-          tar -xzf $src --no-same-owner
-        '';
-      } else { }
-    ));
-}
+    prev.buildPackage (args // {
+      unpackPhase = ''
+        tar -xzf $src --no-same-owner
+      '';
+    });
+}