ci: implement asynchronous riscv64 testing via Snap Store edge channel

[rkt0209]

Overview

This PR resolves the issue of GitHub Actions' 6-hour runner timeouts when attempting to synchronously build and test the riscv64 architecture.

As discussed with @tillkamppeter, since Launchpad automatically builds all architectures (including riscv64) in the background after a commit is pushed, we are decoupling the RISC-V compilation from the synchronous PR checks.

Key Changes

• Unblocked PRs: Removed riscv64 from the main build.yml matrix. PRs will now only gate on the faster architectures (amd64, arm64, and armhf), allowing for rapid merging without waiting up to 4 hours for Launchpad's RISC-V queue.
• Added Edge Monitor (test-riscv-edge.yml): Created a new daily cron workflow that tests the RISC-V build asynchronously.
  • Queries the public Snapcraft API (api.snapcraft.io) for the latest riscv64 CUPS snap in the edge channel.
  • If a new build is detected, it downloads the .snap file.
  • Extracts the binary using squashfs-tools and runs our Ghostscript smoke test locally using qemu-riscv64-static.
  • Gracefully exits if the build is not yet published, preventing false-positive CI failures.

Testing Performed

• Manually triggered test-riscv-edge.yml on my fork. It successfully queried the API, downloaded version 2.4.16-2, unpacked it, and passed the emulated Ghostscript smoke test.