fix: flutter_plugin generates all platform jobs in main workflow

[felangel]

Description
As a developer, I expect the main workflow generated by the template to only include the specified platforms.

[unicoderbot]

Great, I will work on a PR and push it soon.

• Preparing environment
  • Cloning repository
  • Setting up dev environment
• Working on changes
• Reviewing changes
• Opening pull request

I've opened a pull request: #1643