I have found a solution, although I think a better solution should be implemented by the Ansible team. Rename main.yml to tasks.yml, and then write the following to main.yml:
---
- { include: tasks.yml, become: yes }
Another solution is to pass the parameter directly in site.yml, but the main idea of the question was reusing the role in other projects without forgetting it needs root:
---
- hosts: localhost
roles:
- { role: name, become: yes }