riscv64: add minimal CI (compile-only)

[LuckyMagician99]

This PR adds a minimal riscv64 CI job (compile-only).

• QEMU-based build via uraimo/run-on-arch-action (Ubuntu 22.04)
• Calls project's ci/run_build.sh with ARCH_SUFFIX=riscv64
• No behavior change; just CI coverage to catch regressions on riscv64

Verification (native + container):

• Native build on openEuler riscv64: tini version 0.19.0 - git.369448a
• Artifact run under riscv64 container:
  $ docker run --rm --platform=riscv64 -v "$PWD":/work ubuntu:22.04 /work/tini-riscv64 --version
  tini version 0.19.0 - git.2887798
  $ docker run --rm -it --platform=riscv64 -v "$PWD":/work --entrypoint /work/tini-riscv64 ubuntu:22.04 -s -- bash -lc 'echo "bash PID=$$, PPID=$PPID"; cat /proc/1/comm; readlink /proc/1/exe'
  bash PID=7, PPID=1
  tini-riscv64
  /work/tini-riscv64

Context: refs #239 (riscv64 proposal with native build + PID1 experiment).
Happy to switch to cross-compilation, merge into an existing workflow, or adjust triggers per maintainers’ preference.

[LuckyMagician99]

Hi @krallin — friendly ping.

Could you please share guidance on PR #240 (minimal riscv64 compile-only CI)? Since this comes from a fork, GitHub Actions won’t run until a maintainer approves the workflow run. I’m happy to adjust to your preference:

switch to cross-compilation (no QEMU),

fold into an existing workflow, or

even port the check to the repo’s current CI setup if GitHub Actions isn’t preferred.

Scope is intentionally small (no behavior change; compile-only) to catch regressions early. If this is out-of-scope for Tini right now, a quick note on preferred direction (or acceptance criteria) would be super helpful.

Thanks for your time!