lib.strings.trimWith: Refactor scoping

I wanted to use this instead of [my ownimplementation](https://github.com/nix-community/pyproject.nix/blob/65d4134/lib/util.nix#L18-L23) and got a small performance regression.

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.
@@ -425,12 +424,14 @@ 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;
     in
-      optionalString (res != null) (head res);
+    optionalString (res != null) (head res);
 
   /**
     Construct a Unix-style, colon-separated search path consisting of