When testing the main branch cloning viewer fails

[max-nextcloud]

Our github cypress workflow sets an environment variable BRANCH that matches the target branch of the PR - but we use it as the server branch for the container.

This leads to problems when cloning apps that use the master branch such as viewer.

Originally posted by @max-nextcloud in #731 (comment)

[joshtrichards]

Cloning at that point doesn't even work currently since apps/ is read-only at that point the cycle. We'd either need to change the default mounting to RW or do the app cloning in the workflow outside of Cypress itself (i.e. in the init job).