Put slack-desktop on the fast ring

Omarchy is getting an Install > Service > Slack menu entry, and that
needs the package on every channel. slack-desktop has only been built
for edge so far. The fast ring builds it for rc and stable as well,
like the other service packages (Spotify, Dropbox, 1Password, NordVPN).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

pkgbuilds/slack-desktop/.omarchy/package.json

@@ -1,5 +1,6 @@
 {
   "source": "local",
+  "release_ring": "fast",
   "upstream": {
     "watch": {
       "json": "https://slack.com/api/desktop.latestRelease?arch=x64&variant=deb",