build: use lts shorthand in GitHub Actions

Rather than hard-coding GitHub Actions to use Node.js 14.x, use the
`lts/*` shorthand for "most recent LTS version".

PR-URL: https://github.com/nodejs/node/pull/39538


Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>