Context: `fetchdep.sh` attempts to check out a github repository based on the
details in a pull request. To do this, it needs to know how to find the pull
request. So, the github workflows attempt to set environment variables to tell
it. Unfortunately, they currently disagree about what the names of the
environment variables should be.
This appears to have been introduced by #8498.
To simplify matters, we may as well have the script use `${GITHUB_REPOSITORY}`
directly, and remove te unused `REPOSITORY` env var from the workflows.
* Split Cypress out into its own workflow
* Improve PR Details job to use github-script and output labels
* Fix wrongly using github.ref in workflow_run actions which always refer to develop
* Update pr-details to be far more generic
* Tweak how we fill command-prefix
* Tweak cypress job to pass more params & fix if condition
* Bring in external changes
* Add docs
* Use new composite action, and an action to update a status check based on this workflow run
* Iterate approach
* Split Cypress out into its own workflow
* Improve PR Details job to use github-script and output labels
* Fix wrongly using github.ref in workflow_run actions which always refer to develop
* Update pr-details to be far more generic
* Tweak how we fill command-prefix
