Enable CI builds for riscv64-linux (#3685)

Author: yvt

.github/workflows/release.yml

@@ -54,6 +54,11 @@ jobs:
           rust: stable
           target: aarch64-unknown-linux-gnu
           cross: true
+        - build: riscv64-linux
+          os: ubuntu-20.04
+          rust: stable
+          target: riscv64gc-unknown-linux-gnu
+          cross: true
         - build: x86_64-macos
           os: macos-latest
           rust: stable