lib.strings: remove assertMsg usage

assertMsg sends our error message through a function call, even if the
error condition doesn't trigger. This requires a lot of thunk allocation
that can be easily avoided.

lib/strings.nix

@@ -2532,8 +2532,9 @@ rec {
       strw = lib.stringLength str;
       reqWidth = width - (lib.stringLength filler);
     in
-    assert lib.assertMsg (strw <= width)
-      "fixedWidthString: requested string length (${toString width}) must not be shorter than actual length (${toString strw})";
+    assert
+      strw <= width
+      || throw "fixedWidthString: requested string length (${toString width}) must not be shorter than actual length (${toString strw})";
     if strw == width then str else filler + fixedWidthString reqWidth filler str;
 
   /**