Fix project exports clobbering concurrent export paths

When a project export completes, it removes everything in Project#import_export_shared.archive_path, which can erase files needed for another, ongoing project export. This is problematic for custom templates, which exports an existing project to get the most recent changes and imports that archive to another project.

To avoid this from happening, we generate a random unique subpath in the shared temporary directory so that multiple exports can work at the same time.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/14716