installer: better check for /bin/dialog

this works on distros where /bin isn't a symlink to /usr/bin

installer.sh.in

@@ -1425,7 +1425,7 @@ menu() {
     esac
 }
 
-if [ ! -x /bin/dialog ]; then
+if ! command -v dialog >/dev/null; then
     echo "ERROR: missing dialog command, exiting..."
     exit 1
 fi