CI: support ci job on riscv64 hardware

As we have discussed [this]([#t-infra > We're eager to provide the community with RISC-V CI infrastr @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/We're.20eager.20to.20provide.20the.20community.20with.20RISC-V.20CI.20infrastr/near/566193554)) on zulipchat channel, let's focus on some details on how to push this.