Add a Github workflow to build Android RISC-V, when an NDK is available

[prashanthswami]

Similar to our other Android targets in the workflows, we should add an Android RISC-V build target. This isn't currently trivial, because the Android NDK w/RISC-V support is only in canary, so needs to be downloaded off the Android public builders, as opposed to the current workflow actions that expect a stable release.

We could hack in support for that, or alternatively just wait for a formal release to be made.