CI: use openwrt official tools container by default

Use openwrt official tools container by default.
Fork will use openwrt tools container by default.

This can be disabled by setting the option use_openwrt_container to
false for the build.yml and check-kernel-patches.yml.

The push-containers workflow is disabled on forks. The workflow can be
reenabled by commenting the condition in push-containers.yml.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>