lib.lists.foldl': Make strict in the initial accumulator

To maintain backwards compatibility, this can't be changed in the Nix language.
We can however ensure that the version Nixpkgs has the more intuitive behavior.

lib/tests/misc.nix

@@ -524,10 +524,10 @@ runTests {
     expected = [ 3 2 1 ];
   };
 
-  # The same as builtins.foldl', lib.foldl' doesn't evaluate the first accumulator strictly
+  # Compared to builtins.foldl', lib.foldl' evaluates the first accumulator strictly too
   testFoldl'StrictInitial = {
     expr = (builtins.tryEval (foldl' (acc: el: el) (throw "hello") [])).success;
-    expected = true;
+    expected = false;
   };
 
   # Make sure we don't get a stack overflow for large lists