ci: add riscv64gc-unknown-linux-gnu to release artifacts (#1568)

Add cross-compilation support for riscv64gc-unknown-linux-gnu, following
the same Docker-based pattern as the existing aarch64-linux target.

Fixes #1567

Author: gounthar

.github/workflows/main.yml

@@ -30,6 +30,11 @@ jobs:
           env:
             CARGO_BUILD_TARGET: aarch64-unknown-linux-gnu
             DOCKER_IMAGE: ./ci/docker/aarch64-linux/Dockerfile
+        - build: riscv64gc-linux
+          os: ubuntu-latest
+          env:
+            CARGO_BUILD_TARGET: riscv64gc-unknown-linux-gnu
+            DOCKER_IMAGE: ./ci/docker/riscv64gc-linux/Dockerfile
 
         - build: x86_64-macos
           os: macos-latest

ci/docker/riscv64gc-linux/Dockerfile

@@ -0,0 +1,7 @@
+FROM ubuntu:18.04
+
+RUN apt-get update -y && apt-get install -y gcc gcc-riscv64-linux-gnu ca-certificates git
+
+ENV PATH=$PATH:/rust/bin
+ENV CARGO_BUILD_TARGET=riscv64gc-unknown-linux-gnu
+ENV CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc