x/build/env/linux-riscv-qemu: add new RISC-V builder under qemu #30262
Description
@rminnich @josharian et al want to start landing RISC-V code, so we need a builder.
@rminnich says qemu (from the sifive fork) is good now.
We can run a builder similar to what we're going to do for Fuchsia and what we previous did for a bit in https://github.com/golang/build/tree/master/env/linux-arm-qemu (but that was more emulation than we'd probably want here). We'll probably want to only run tests in qemu but do builds natively.