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.