Find a better way to add the EBRAINS auth. token in Github Actions

To run the Siibra tests using Github Actions, we are currently storing an EBRAINS token inside a Github Secret. The problem is that the token expires after 7 days, so it need to be manually updated each week.

An idea to overcome this is to write a script (which will run inside a Github Actions workflow before running the tests), which will make a call to the authorization endpoint nexus-iam.humanbrainproject.org/v0/oauth2/authorize  and will receive a valid EBRAINS token. For this call a username and a password should be provided. The password and optionally the username could be stored as Github Secrets.

The EBRAINS token obtained using said script will be stored as an env variable and used in the Siibra tests.