fix pkg install status check with apt-cache and mention pbs-client repos

pbc

@@ -127,11 +127,14 @@ function install_proxmox_backup_client() {
     return 0
   fi
 
-  if apt-get search proxmox-backup-client &>/dev/null; then
+  if apt-cache show proxmox-backup-client &>/dev/null; then
     apt_get_install proxmox-backup-client
     return $?
   fi
 
+  echo "Info: proxmox-backup-client is not available from any configured apt repository"
+  echo "Info: add the Proxmox pbs-client repository to install a signed, apt-managed package"
+
   $update && local action='Updating' || local action='Installing'
   echo "$action proxmox-backup-client-static binary ..."
 
@@ -262,7 +265,7 @@ if [ "$action" == 'install' ]; then
     source "$cfgfile" || exit $?
   fi
   PBC=${PBC:-/usr/local/bin/proxmox-backup-client}
-  for exe in apt-get dpkg dpkg-deb curl; do
+  for exe in apt-cache apt-get dpkg dpkg-deb curl; do
     ! command -v "$exe" >/dev/null && echo "Error: $exe executable not found!" >&2 && exit 200
   done
 else