[action] [PR:18015] fix the compile issue for slim image (#18015)

[mssonicbld]

Why I did it
The PR introduced a bug for slim image build, #17905, by which the sonic_asic_platform is missing when build docker image for slim image.

[ building ] [ target/docker-dhcp-relay.gz ]
/sonic/dockers/docker-dhcp-relay/cli-plugin-tests /sonic
/sonic
Traceback (most recent call last):
File "/usr/local/bin/j2", line 8, in
sys.exit(main())
File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 202, in main
output = render_command(
File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 186, in render_command
result = renderer.render(args.template, context)
File "/usr/local/lib/python3.9/dist-packages/j2cli/cli.py", line 85, in render
return self._env
File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 1090, in render
self.environment.handle_exception()
File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 832, in handle_exception
reraise(*rewrite_traceback_stack(source=source))
File "/usr/lib/python3/dist-packages/jinja2/_compat.py", line 28, in reraise
raise value.with_traceback(tb)
File "/sonic/dockers/docker-dhcp-relay/Dockerfile.j2", line 48, in top-level template code
{% if build_reduce_image_size != "y" or sonic_asic_platform != "broadcom" %}
jinja2.exceptions.UndefinedError: 'sonic_asic_platform' is undefined
make: *** [slave.mk:1072: target/docker-dhcp-relay.gz] Error 1
make: *** Waiting for unfinished jobs....
[ finished ] [ target/docker-swss-layer-bullseye.gz ]
[ finished ] [ target/docker-syncd-brcm-dnx.gz ]
make[1]: *** [Makefile.work:608: target/sonic-broadcom.bin] Error 2
make[1]: Leaving directory '/data/work/1/s'
make: *** [Makefile:41: target/sonic-broadcom.bin] Error 2
And why it slipped the PR test? PR test doesn't compile with slim option, it won't check sonic_asic_platform != "broadcom" for PR build.

Work item tracking
Microsoft ADO (number only):
How I did it
Export sonic_asic_platform for docker build in slave.mk

How to verify it
build with slim image option.

[mssonicbld]

Original PR: #18015