ci: Enable build-check for runtime on riscv64

`runtime` support for riscv64 is now ready, let enable building and
testing on that component.

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

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

@@ -43,6 +43,11 @@ jobs:
               - rust
               - musl-tools
               - protobuf-compiler
+          - name: runtime
+            path: src/runtime
+            needs:
+              - golang
+              - XDG_RUNTIME_DIR
           - name: runtime-rs
             path: src/runtime-rs
             needs: