Add riscv64gc-unknown-linux-gnu to release binaries

[gounthar]

Summary

wkg currently ships release binaries for x86_64 and aarch64 on Linux, macOS, and Windows. This issue requests adding riscv64gc-unknown-linux-gnu to the release matrix.

Motivation

RISC-V adoption is growing, with boards like Banana Pi F3 (SpacemiT K1, rv64gc) running mainstream Linux. wkg is the Wasm package manager used alongside wasm-tools and cargo component. Having pre-built riscv64 binaries lets developers on RISC-V hardware use wkg without building from source.

Evidence

• Fork with riscv64 builds: gounthar/wasm-pkg-tools produces riscv64 release binaries via cross on GitHub Actions — see v0.15.0 release.
• Native build verified on Banana Pi F3 (rv64gc, Armbian Linux trixie).
• wkg uses rustls (no OpenSSL dependency), making cross-compilation straightforward.

Proposed approach

The existing CI already uses cross for aarch64-unknown-linux-gnu. Adding riscv64 is a one-line matrix addition in both publish_dev_release and publish_tagged_release jobs in .github/workflows/publish-binaries.yml:

- rust-target: riscv64gc-unknown-linux-gnu
  os: ubuntu-latest
  cross: true

A Cross.toml entry may not be needed since wkg uses rustls and has no native C dependencies requiring special linking.

I'm happy to open a PR implementing this if maintainers are open to the addition.