De-deprecate {{env.VARIABLE}} source

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

I originally deprecated this when I added the !env chain source, but if all you're doing is reading a static variable, this is a simple alternative. It costs very little to continue to support this so I think it's worth keeping around.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

Higher complexity with more features supported.

QA

How did you test this?

Doc change only

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked