Cache ISO mirror by channel instead of by date

The previous per-day cache (iso_YYYY-MM-DD) forced a fresh ~6GB download
on the first build of every day. Different channels (stable/edge/rc)
HAVE different package sets and shouldn't share a cache, but different
DAYS within a channel don't, so keying by channel gives us a stable
cache that persists indefinitely.

omarchy* packages always rebuild fresh inside the container regardless:
build-omarchy-packages.sh nukes /tmp/omarchy-pkg-build at the start
and the final mv overwrites the cached .pkg.tar.zst (pkgver is constant).
--no-cache still bypasses the mount entirely for a clean-slate run.

bin/omarchy-iso-make

@@ -102,13 +102,16 @@ if [ -d "/var/cache/pacman/pkg" ]; then
   DOCKER_ARGS+=(-v "/var/cache/pacman/pkg:/var/cache/pacman/pkg")
 fi
 
-# Mount the build cache directory where packages are downloaded
+# Mount the build cache directory where packages are downloaded. Keyed by
+# channel so stable/edge/rc don't pollute each other but the cache persists
+# across days (different channels have different package sets; days don't).
+# omarchy* packages always rebuild fresh inside the container regardless of
+# what's in the cache (build-omarchy-packages.sh nukes its work dir and the
+# final mv overwrites the cached .pkg.tar.zst). --no-cache bypasses the mount
+# entirely if you want a truly clean download.
 if [[ -z "$NO_CACHE" ]]; then
-  # using a dated dir on the host to refresh daily if building multiple times
-  BUILD_DATE=$(date +%Y-%m-%d)
-  OFFLINE_REPO_BUILD_CACHE_DIR="$HOME/.cache/omarchy/iso_${BUILD_DATE}/airootfs/var/cache/omarchy"
+  OFFLINE_REPO_BUILD_CACHE_DIR="$HOME/.cache/omarchy/iso_${OMARCHY_MIRROR}/airootfs/var/cache/omarchy"
   mkdir -p "$OFFLINE_REPO_BUILD_CACHE_DIR"
-  # Mount to /var/cache/airootfs/var/cache/omarchy since that's where repo packages go in the ISO
   DOCKER_ARGS+=(-v "$OFFLINE_REPO_BUILD_CACHE_DIR:/var/cache/airootfs/var/cache/omarchy")
 fi