ci: Bring in riscv toolchain as well

Make sure we have toolchains for all of the supported arches.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

.github/workflows/build.yml

@@ -34,8 +34,7 @@ jobs:
         with:
           app-path: zephyr-rust-lang
           manifest-file-name: ci-manifest.yml
-          toolchains: arm-zephyr-eabi
-            # TODO: Also bring in riscv.
+          toolchains: arm-zephyr-eabi:riscv64-zephyr-elf
 
       - name: Install Rust Targets
         shell: bash