Offical response from GitHub Support below.
Thank you for reaching out to GitHub Support!
While we don’t have a way to share environment variables across workflows yet, if these values aren’t dynamically generated, would using repository secrets be an option?
There are some limitations to note:
As mentioned, this wouldn’t work if the values are dynamically generated – you’d have to input these hardcoded values as secrets before the workflow runs
A workflow can only have up to 100 secrets
A repository can only have 100 secrets
Secrets are limited to 64 KB in size
Again we apologize that we don’t have a direct solution for this at this time.
However, our roadmap is now publicly visible, so we recommend that you keep an eye on the GitHub Blog and roadmap for the latest announcements about new features.