Add backup name env var handling in restore CI task

[petechd]

What is the context of this PR?

Currently we don't handle situations where the BACKUP_NAME env var is not provided (and there is no default hardcoded). The script tries to extract export metadata with no backup name appended to it and it fails but the error is not clear. This updates the script so it warns that the var needs to be set.

How to review

Fly execute the restore_questionnaire_state.yaml file, without providing the BACKUP_NAME var and check if it errors and you get the "BACKUP_NAME variable must be set" message. Fly execute with BACKUP_NAME and check if you don't get the same error.

Checklist

• New static content marked up for translation
• Newly defined schema content included in eq-translations repo

[petechd]

Fix restore_questionnaire_state script backup_name env var handling