fix(systemd-start): /usr/bin/emqx is bash not sh

deploy/packages/deb/debian/emqx.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=emqx daemon
+After=network.target
+
+[Service]
+User=emqx
+Group=emqx
+Type=forking
+Environment=HOME=/var/lib/emqx
+ExecStart=/usr/bin/emqx start
+LimitNOFILE=1048576
+ExecStop=/usr/bin/emqx stop
+Restart=on-failure
+RestartSec=5s
+
+[Install]
+WantedBy=multi-user.target