Observe flutter tags with git

[PlugFox]

Build specific version at major versions.

git -c 'versionsort.suffix=-' ls-remote --exit-code --refs --sort='version:refname' \
  --tags https://github.com/flutter/flutter.git '*.*.*' \
  | cut -d '/' -f 3 \
  | grep -e "^[0-9]\+\.[0-9]\+\.[0-9]\+$" \
  | tail -n 1