Build fast-ring for rc as rc, instead of copying stable's artifacts

The rc channel's Arch base can sit anywhere between stable's snapshot and
edge's, so a package built against stable's libraries is not necessarily
correct for rc. Copying stable's fast-ring artifacts into rc therefore shipped
possibly-mislinked packages to RC testers. Fast-ring packages now build
natively for all three channels, each in its own image against its own base
mirror, and the stable release's replication step is gone.

That required separating 'may be built here' from 'whose version wins'. The
release pair is now marked "pinned": its version is set per release on the rc
branch, so it builds for rc only from that branch's worktree
(OMARCHY_RC_PINS=1, set by omarchy-release rc) — master's shipped pins can
never overwrite an in-flight RC, even though check-versions now discovers rc
work like it does for edge and stable.

systemd/omarchy-auto-release-rc.service

@@ -5,13 +5,13 @@ Wants=network-online.target
 
 [Service]
 Type=oneshot
-# Runs from the main checkout: bin/auto-release-rc creates and syncs the rc
-# branch worktree itself, so a host with no rc branch yet is a no-op instead
-# of a unit that fails every five minutes. Builds happen in the worktree but
-# publish into the primary checkout's channel tree via OMARCHY_REPO_ROOT.
-ExecStart=/bin/bash -c 'source /root/.omarchy/build-credentials && /root/omarchy-pkgs/bin/auto-release-rc'
+# Runs from the main checkout, exactly like edge and stable: this builds the
+# packages the version check discovers for rc (fast-ring), natively in the rc
+# image against the rc mirror. The pinned release pair is NOT built here —
+# its version comes from the rc branch, and omarchy-release builds it in that
+# branch's worktree with OMARCHY_RC_PINS=1.
+ExecStart=/bin/bash -c 'source /root/.omarchy/build-credentials && /root/omarchy-pkgs/bin/auto-release rc'
 Environment=OMARCHY_STATE_DIR=/root/.state
-Environment=OMARCHY_REPO_ROOT=/root/omarchy-pkgs/pkgs.omarchy.org
 TimeoutStartSec=7200
 
 [Install]