ci: add build coverage for riscv64

ci: add build coverage for riscv64

This is already a primary architecture in Ubuntu, and more distributions
are adding support for it. It's too slow for emulation, but we can at
least verify that compilation works.

Use the arm worker, for two reasons:

- it is already set up with ports.ubuntu.com so we don't have to muck
  with apt sources manually
- it is used a lot less than the x86 worker