CI: use new `openwrt` Docker username

We now own `openwrtorg` and `openwrt`, where the latter replaces the
former. Slowly migrate over.

Signed-off-by: Paul Spooren <mail@aparcar.org>

.github/workflows/Dockerfile

 ARG ARCH=x86-64
-FROM openwrtorg/rootfs:$ARCH
+FROM openwrt/rootfs:$ARCH
 
 ADD entrypoint.sh /entrypoint.sh