fix Gitpod PR button

[AngryMaciek]

Open in Gitpod button does not appear on PRs... again...

[github-actions]

Hey @AngryMaciek,

Thanks for taking interest in the project and the effort to help us improve it 👍
Project lead will probably come around and assign proper labels shortly...

Actually... maybe you would like to work on this issue yourself?
You may start with reading our contributing instructions.

Your help is much appreciated!