rust: install rustfmt for riscv32 as well

With the above rust arch fixes it builds just fine.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f417ae30)
Signed-off-by: Steve Sakoman <steve@sakoman.com>