osbuild-composer: don't use hardcoded state directory

Use $STATE_DIRECTORY environment variable which is set by systemd
because we use: StateDirectory=osbuild-composer in the service unit.

also change systemd unit to include STATE_DIRECTORY, because
RHEL comes with older systemd version, so we need to set this variable explicitly.

cmd/osbuild-composer/main.go

@@ -54,7 +54,10 @@ func main() {
 	flag.BoolVar(&verbose, "v", false, "Print access log")
 	flag.Parse()
 
-	stateDir := "/var/lib/osbuild-composer"
+	stateDir, ok := os.LookupEnv("STATE_DIRECTORY")
+	if !ok {
+		log.Fatal("STATE_DIRECTORY is not set. Is the service file missing StateDirectory=?")
+	}
 
 	listeners, err := activation.ListenersWithNames()
 	if err != nil {