pkgs/phps: Fix type error

configureFlags should take array but lib.optionalString will return
a string when the condition is false. It only ever worked because
we always run the code on PHP < 7.4 and lib.optionalString returns
the argument as is when the condition is false.

Author: jtojnar

pkgs/phps.nix

@@ -43,7 +43,7 @@ let
 
           configureFlags =
             attrs.configureFlags
-            ++ prev.lib.optionalString (prev.lib.versionOlder args.version "7.4") [
+            ++ prev.lib.optionals (prev.lib.versionOlder args.version "7.4") [
               # phar extension’s build system expects hash or it will degrade.
               "--enable-hash"
             ];