Annotations using environment files

[alcuadrado]

Describe the enhancement
I think it would be valuable to be able to set an annotation without it being printed to stdout, as currently, they can lead to duplicated output.

For example, I'm working on a node:test reporter that outputs failures for the user in the CLI and also annotates the PRs when running on GitHub actions. I use core.error() for the latter, but that leads to duplicated error messages.

If I could write to an environment file instead, it would lead to a tidier experience.

Additional information
There are other pieces of functionality that can work either by printing output or by concatenating commands into a file whose path is available as an env variable. That mechanism may be reusable for annotations.