util/gen_xmlpool: Don't write via shell redirection

util/gen_xmlpool: Don't write via shell redirection

Using shell redirection to write to a file is more complicated than
necessary, and has the potential to run into unicode encoding problems.
It's also less code.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108530

v2: - update commit message to say less about LANG=C
    - use flags instead of positional arguments for the script (Emil)

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>