Fix the "path" option type

Comparing a string with a context with "/" doesn't work because the
"==" operator also compares context.  So the context needs to be
discarded.

pkgs/lib/types.nix

@@ -89,7 +89,7 @@ rec {
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.
-      check = lib.traceValIfNot (x: builtins.substring 0 1 (toString x) == "/");
+      check = lib.traceValIfNot (x: builtins.unsafeDiscardStringContext (builtins.substring 0 1 (toString x)) == "/");
     };
 
     listOf = types.list;