[CI] Comment on new env PRs

[burtenshaw]

This PR update the CI for new PRs on envs. It does two things:

• validates the env with the ci and tells contributors to push to the hub.
• does not use any secrets.

Here is a succesful comment on a fork pr.

⚠️ This PR will need to follow #160 because it depends on the validate command.

[zkwentz]

Reviewed your changes atop #160. Looks good.

One thought: in the comment can we note the preferred flow, something like:

Your env passes the vibe check. However, most environments should go straight to the hub, they will automatically be added to the official Env Hub collection on a nightly basis. Environments in the official specification repo are only meant to demonstrate usage of a specific spec feature for educational purposes.

[zkwentz]

gucci