Update uptimekuma-install.sh: Install npm

The script currently fails running "npm run setup", since npm is not installed. This patch adds npm installation.

Tested by running the script manually on an LXC container.

install/uptimekuma-install.sh

@@ -33,6 +33,10 @@ $STD apt-get update
 $STD apt-get install -y nodejs
 msg_ok "Installed Node.js"
 
+msg_info "Installing npm"
+$STD apt-get install -y npm
+msg_ok "Installed npm"
+
 msg_info "Installing Uptime Kuma"
 $STD git clone https://github.com/louislam/uptime-kuma.git
 mv uptime-kuma /opt/uptime-kuma