Add riscv64 support to Alpine

The last version of Alpine prior to support of riscv64 has been dropped. Let's add riscv64 to Alpine docker-rust images.

Author: eshattow

x.py

@@ -62,6 +62,7 @@ def arches(self) -> list[DebianArch]:
     AlpineArch("amd64", "x86_64", "linux/amd64", "x86_64-unknown-linux-musl"),
     AlpineArch("arm64v8", "aarch64", "linux/arm64", "aarch64-unknown-linux-musl"),
     AlpineArch("ppc64le", "ppc64le", "linux/ppc64le", "powerpc64le-unknown-linux-musl"),
+    AlpineArch("riscv64", "riscv64", "linux/riscv64", "riscv64gc-unknown-linux-musl"),
 ]
 
 latest_alpine_version = "3.24"