Review fix-up: align the riscv64 job with the other Linux jobs

- Pin actions/checkout and upload-artifact to the SHAs the other jobs use
  (v7.0.1, v7.0.2); add rust-cache; use the same version-override command.
- Run on ubuntu-24.04 (ubuntu-26.04 is not a known hosted label) and take the
  ports suite from /etc/os-release; 22.04's riscv64 ports lag its amd64
  updates and the multiarch install fails there.
- Add linux-riscv64 to the release job's needs so release-linux-riscv64 is
  downloaded before the draft release is built.
- Drop the unused libasound2 dependency, the linker -L flags and the
  qemu-static symlink; package.sh's emulator check is the smoke test
  (QEMU_LD_PREFIX), and quote $EMULATOR there.
- Move the job above the FreeBSD comment; list riscv64 in docs/releasing.md.

Verified: the job's steps in an ubuntu:24.04 container build, package and
run gridcraft-cli --version under qemu-riscv64.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/linux/package.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Build and package GridCraft for Linux (<arch> is x86_64 or aarch64):
+# Build and package GridCraft for Linux (<arch> is x86_64 or aarch64, or riscv64 when cross-compiling):
 #
 #   $DIST/gridcraft-<version>-linux-<arch>.AppImage  any distro with glibc >= the build host's
 #   $DIST/gridcraft-<version>-linux-<arch>.AppImage.zsync  delta updates (needs zsyncmake)
@@ -143,7 +143,7 @@ fi
 if [ -z "${CROSS_TARGET:-}" ]; then
   "$STAGE/usr/bin/gridcraft-cli" --version
 elif [ -n "${EMULATOR:-}" ]; then
-  $EMULATOR "$STAGE/usr/bin/gridcraft-cli" --version
+  "$EMULATOR" "$STAGE/usr/bin/gridcraft-cli" --version
 fi
 echo "==> done"
 ls -lh "$DIST"