lib.strings.trimWith: Refactor scoping (#339001)

lib/strings.nix

@@ -408,7 +408,6 @@ rec {
       start ? false,
       end ? false,
     }:
-    s:
     let
       # Define our own whitespace character class instead of using
       # `[:space:]`, which is not well-defined.
@@ -425,7 +424,9 @@ rec {
           "(.*[^${chars}])[${chars}]*"
         else
           "(.*)";
-
+    in
+    s:
+    let
       # If the string was empty or entirely whitespace,
       # then the regex may not match and `res` will be `null`.
       res = match regex s;