I realize this task by using two files robots-open.txt and robot-closed.txt and setting ENV variable.
Ad then add WebPackPlugin into quasar.config.js as written above:

extendWebpack(cfg) {
      const robotsMode =
        process.env.OPEN_FOR_ROBOTS === 'true' ? 'opened' : 'closed'
      console.log(`==> Robots.txt are ${robotsMode}`)
      cfg.plugins.push(
        new CopyWebpackPlugin({
          patterns: [
            {
              from: `robots/robots-${robotsMode}.txt`,
              to: 'robots.txt',
            },
          ],
        }),
      )
    },