ci: Enable runtime-rs component build-check on riscv64

`runtime-rs` is now buildable and testable on riscv64 platforms, enable
`build-check` on `runtime-rs`.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

.github/workflows/build-checks-preview-riscv64.yaml

@@ -43,6 +43,10 @@ jobs:
               - rust
               - musl-tools
               - protobuf-compiler
+          - name: runtime-rs
+            path: src/runtime-rs
+            needs:
+              - rust
 
     steps:
       - name: Adjust a permission for repo