rebuildctl pkgs sync: Storing a different distro value then the sync method used

[kpcyrd]

This command:

% rebuildctl pkgs sync --print-json archlinux cachyos-v3 'https://mirror.cachyos.org/repo/$arch/$repo' --architecture x86_64_v3

Produces entries like this:

  {
    "name": "cachy-browser",
    "version": "94.0.2-3",
    "distro": "archlinux",
    "suite": "cachyos-v3",
    "architecture": "x86_64_v3",
    "input_url": null,
    "artifacts": [
      {
        "name": "cachy-browser",
        "version": "94.0.2-3",
        "url": "https://mirror.cachyos.org/repo/x86_64_v3/cachyos-v3/cachy-browser-94.0.2-3-x86_64_v3.pkg.tar.zst"
      }
    ]
  },

Since distro is set to archlinux it'd pick the Arch Linux rebuilder-backend. Changing this to cachyos fails like this:

% rebuildctl pkgs sync --print-json cachyos cachyos-v3 'https://mirror.cachyos.org/repo/$arch/$repo' --architecture x86_64_v3
Error: No integrated sync for "cachyos", use sync-stdin instead

There should be a way to use the built-in sync code for Arch Linux while setting distro to a different value.

Related to #107