Correctly detect if CentOS is present

download_boot_systems.sh

@@ -17,9 +17,12 @@ then
 fi
 }
 download_centos_8_if_needed() {
-if [ ! -f version.info ]
+if [ ! -f centos-installer/.treeinfo ]
 then
+  if [ ! -f CentOS-8.2.2004-x86_64-minimal.iso ]
+  then
     wget http://mirrors.neterra.net/centos/8.2.2004/isos/x86_64/CentOS-8.2.2004-x86_64-minimal.iso
+  fi
   mkdir -p centos-installer
   mount -o loop CentOS-8.2.2004-x86_64-minimal.iso centos-installer
 fi