Add missing riscv64gc-unknown-linux-musl entry to dist-workspace (#18686)

This looks omitted from #18228 and I
noticed a warning from the new release step from
#18625

Author: zanieb

dist-workspace.toml

@@ -30,6 +30,7 @@ targets = [
     "powerpc64-unknown-linux-gnu",
     "powerpc64le-unknown-linux-gnu",
     "riscv64gc-unknown-linux-gnu",
+    "riscv64gc-unknown-linux-musl",
     "s390x-unknown-linux-gnu",
     "x86_64-unknown-linux-gnu",
     "x86_64-unknown-linux-musl",