Only run HA CLI interactively if stdout is a terminal

Flags for running HA CLI commands in an interactive shell added in #3238
cause the command to fail if the process is not running in a terminal.
This is needed for example for the fsfreeze hook, otherwise the command
fails, as seen in this trace when the hook is executed:

-----------
+ '[' thaw '=' freeze ]
+ '[' thaw '=' thaw ]
+ echo 'File system thaw requested, thawing Home Assistant'
File system thaw requested, thawing Home Assistant
+ ha backups thaw
the input device is not a TTY
------------

However, for example on Proxmox this message is not logged anywhere and
the hook just fails silently (i.e. it doesn't cause the backup to fail).

Fixes #3251

buildroot-external/rootfs-overlay/usr/bin/ha

@@ -3,4 +3,9 @@
 # HA utility
 # ==============================================================================
 
-docker exec -it hassio_cli ha "$@"
+if [ -t 1 ]; then
+	# stdout (fd 1) must be terminal, otherwise `docker exec -ti` fails
+	docker exec -it hassio_cli ha "$@"
+else
+	docker exec hassio_cli ha "$@"
+fi