.github: cross-compile riscv64 Linux asset (#798)

Continue the recent `zig cc` work [1], such that the next configlet
release will have two new release assets:

    configlet_4.0.0-beta.14_linux_riscv64.tar.gz
    configlet_4.0.0-beta.14_linux_riscv64.tar.gz.minisig

where the archive contains the executable:

    $ file ./configlet
    ./configlet: ELF 64-bit LSB executable, UCB RISC-V, RVC, double-float ABI, version 1 (SYSV), statically linked, stripped

The riscv64-linux-musl target will have Tier 1 Zig support [2].

Refs: #24

[1] 0e8d665, ".github, config: use Zig to cross-compile arm64 Linux asset", 2023-08-13
[2] https://ziglang.org/download/0.11.0/release-notes.html#Support-Table

.github/bin/cross-compile

@@ -8,7 +8,15 @@ zig_target="${ZIG_TARGET}"
 
 cross_compile() {
   local target="$1"
-  local arch='arm64'
+
+  local zig_arch
+  zig_arch="$(cut -d'-' -f1 <<< "${target}")"
+  local arch
+  case "${zig_arch}" in
+    aarch64) arch='arm64'       ;;
+    *)       arch="${zig_arch}" ;;
+  esac
+
   local os
   os="$(cut -d'-' -f2 <<< "${target}")"
   local nim_os

.github/workflows/build.yml

@@ -81,6 +81,9 @@ jobs:
           - runs-on: ubuntu-22.04
             zig_target: aarch64-linux-musl
 
+          - runs-on: ubuntu-22.04
+            zig_target: riscv64-linux-musl
+
           - runs-on: macos-12
             zig_target: aarch64-macos-none