feat: generate a new environment variable with the application's url

[ghusse]

This is needed by future versions of forest-express

Related to https://github.com/ForestAdmin/forest-express/pull/523

Linked to CU-96whq6

Pull Request checklist:

• [x] Write an explicit title for the Pull Request, following Conventional Commits specification
• [x] Create automatic tests
• [x] No automatic tests failures
• [x] Test manually the implemented changes
• [x] Review my own code (indentation, syntax, style, simplicity, readability)
• [x] Wonder if you can improve the existing code

[forest-bot]

Task linked: CU-96whq6 Story #2 - Initialise OpenId support on the express agent and the frontend

[forest-bot]

:tada: This PR is included in version 3.10.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: