[build/concurrency] Only allow one musl-build-image job.

Two musl-build-image jobs can be triggered by two commits done within a
short amount of time. Since building the image times can vary; the job
with the previou commit might finish later; updating the docker hub with
an older commit.

This uses the `concurrency` directive to fix the issue. If another job
is running; they get cancelled and only the latest one spawned is run.