Since we always build musl kata-agent, there is no need to build
it inside a musl container. We can just build on the host and then
copy the binary to the target rootfs.
There are still a lot to clean up and it should be made so for ALL
target distros instead of just alpine. But this is at least working
for alpine first.
Signed-off-by: Peng Tao <bergwolf@hyper.sh>
