Use right Gitlab CI var for Cypress branch name

Thank you for making a Merge Request. Before you merge this, please make sure that you:

• Updated chart version number in Chart.yaml
• Updated the CHANGELOG.md
• Manually tested chart changes with the stackspin bootstrap script
• Created an MR in stackspin/stackspin that applies the changes from this MR
• Tests in that MR in stackspin/stackspin succeed