yarn: fix steamos-update exit code — 7 means no update, not 0

Steam interprets exit 0 from 'steamos-update check' as 'update applied
successfully' and shows a persistent 'update available' notification.
The SteamOS convention is exit 7 = no update available.

hosts/yarn/default.nix

@@ -174,7 +174,7 @@
           CURRENT=$(readlink -f /nix/var/nix/profiles/system)
           if [ "$CURRENT" = "$STORE_PATH" ]; then
             >&2 echo "[steamos-update] no update available"
-            exit 0
+            exit 7
           fi
 
           # check-only mode: just report that an update exists