CI: Add RISC-V runner support and matrix-based workflow

[whileone-sk]

This PR updates the GitHub Actions CI workflow to support RISC-V builds
using a matrix-driven runner setup.

Key changes:

• Introduces riscv64 support via workflow_dispatch inputs
• Dynamically generates job matrices based on requested runner counts
• Reuses existing x86_64 runner registration for RISC-V (QEMU/emulation-based)
• Keeps Windows and macOS matrices unchanged
• Validated locally using act with a custom event.json

Notes:

• Local-only files (event.json, act config) are committed.
• Some cache and cleanup steps are skipped or behave differently under act

[BtbN]

This is not something we need or want (It'd be way to slow).
And we also won't be pulling random third party images for our CI.
Also, you're aware that this is only backend for our Forgejo CI?