Instance-level action variables

[jbgomond]

Feature Description

Hello,

Following #27373, it would be great if we could have action variables at an instance-level, alongside secrets.

The use-case is still the same, being able to set variables that applies to all repositories on a given instance, and being able to redefine them at a user / org / repo level.

I'm willing to work on it if approved.

Thanks

Screenshots

No response