modules/buildinfo: simplify implementation

turns out there is a string-slicing function, I just overlooked it when
writing this file (it's even a builtin). So let's use that instead.

modules/buildinfo.nix

@@ -2,20 +2,15 @@
 
 let
   self = sources.self;
-  slice = string: start: length: with lib;
-    strings.concatStrings
-      (lists.take length
-        (lists.drop start
-          (strings.stringToCharacters string)));
 
-  formatDate = date:
+  formatDate = date: with lib.strings;
     let
-      year = slice date 0 4;
-      month = slice date 4 2;
-      day = slice date 6 2;
-      hour = slice date 8 2;
-      minute = slice date 10 2;
-      second = slice date 12 2;
+      year = substring 0 4 date;
+      month = substring 4 2 date;
+      day = substring 6 2 date;
+      hour = substring 8 2 date;
+      minute = substring 10 2 date;
+      second = substring 12 2 date;
     in
       "${year}-${month}-${day} ${hour}:${minute}:${second} UTC";
 in