nixos/testing: fix pyflakes builtins with bash 5.3p0

Printing the file contents with `< file` does not seem to work anymore.

nixos/lib/testing/driver.nix

···
 
           PYFLAKES_BUILTINS="$(
             echo -n ${lib.escapeShellArg (lib.concatStringsSep "," pythonizedNames)},
-            < ${lib.escapeShellArg "driver-symbols"}
+            cat ${lib.escapeShellArg "driver-symbols"}
           )" ${hostPkgs.python3Packages.pyflakes}/bin/pyflakes $out/test-script
         ''}