[Debian-salsa-ci] Custom arm64 and riscv64 flakiness

[Otto Kekäläinen]

Hi Santiago!

I've noticed that the arm image builds for the Salsa CI's own pipeline
frequently fail. They seem to get stuck, and rerunning them one-by-one
seems to get them run all the way and pass.

When you have time, could you please check what is the status with the
arm runner? Maybe the concurrency should be decreased so jobs don't
get stuck?

- Otto