Use project path instead of name in integration branch name

Usually project name and path are identical after project creation. It
is possible, though, to change the name of a project without changing
its path.
When triggering a pipeline on the integration branch, we are using the
CI_PROJECT_NAME variable to reference the project. Unexpectedly, this
variable contains the *path*, not the *name* of the project (see
https://gitlab.com/gitlab-org/gitlab/-/issues/20030).
Hence we have to use the project path in all other places where the
integration branch name is constructed in our Python code.