CI: Switch running the nokia_rx51 test with in-container toolchain

Instead of fetching an arm toolchain to use, run the test with the one
that's already in the container image.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>