tests/functional: use script flags compatible with macOS

Co-authored-by: John Ericson <git@JohnEricson.me>

tests/functional/package.nix

@@ -11,6 +11,7 @@
   git,
   mercurial,
   util-linux,
+  unixtools,
 
   nix-store,
   nix-expr,
@@ -54,6 +55,7 @@ mkMesonDerivation (
         jq
         git
         mercurial
+        unixtools.script
       ]
       ++ lib.optionals stdenv.hostPlatform.isLinux [
         # For various sandboxing tests that needs a statically-linked shell,