Allow multi platform build to be sequential

It's already reported in #621 that the multi platform build fails with OutOfMemory error.

I noticed on the log that the build for both platform is running in parallel

#11 [linux/amd64 cabaldependency 4/5] RUN cabal update
#11 80.82 HEAD is now at a65c29b6a Merge #3801
#11 81.27 Cloning into '/app/dist-newstyle/src/project-3b55de0732f6b166'...
#11 ...
#14 [linux/arm64 cabaldependency 4/5] RUN cabal update
#14 77.43 HEAD is now at 7497a29 Rename forked package
#14 77.62 Cloning into '/app/dist-newstyle/src/ouroboros_-268d3b2310e5b441'...
#14 ...

I tried configuring jobs.<job_id>.strategy.max-parallel but it has no effect.

build-push-action should either respect it or make it possible to set the parallelism for build