firefox: fix wrong syntax grammar for search setting isAppProvided (#6556)

The or operator is left associative, and since there is another
argument after the first term, the interpreter tries to apply whatever
the or-expression evaluates to.  If the first operand is unset, it
evaluates to removeAttrs, and everything is fine, but if it is set to
a boolean (which is what it should be set to), then it tries to apply
a boolean to arguments, and we get a type error.  Bracketing
explicitly with parentheses fixes this.

It presumably went unnoticed because not many people have tried
setting the option
`programs.firefox.profiles.<profile>.search.engines.<engine>.isAppProvided`.

modules/programs/firefox/profiles/search.nix

@@ -48,8 +48,8 @@ let
     let
       requiredInput = {
         inherit name;
-        isAppProvided = input.isAppProvided or removeAttrs input [ "metaData" ]
-          == { };
+        isAppProvided =
+          input.isAppProvided or (removeAttrs input [ "metaData" ] == { });
         metaData = input.metaData or { };
       };
     in if requiredInput.isAppProvided then