Ensure compatibility with ansible 2.23: Jinja constant strings should not contain embedded templates

[max-nuding]

Ansible warns me when running my playbook:

Origin: ~/.ansible/collections/ansible_collections/prometheus/prometheus/roles/prometheus/tasks/preflight.yml:81:25

79 - name: Discover latest version
80   ansible.builtin.set_fact:
81     prometheus_version: "{{ (lookup('url', 'https://api.github.com/repos/{{ _prometheus_repo}}/releases/latest', h...
                           ^ column 25

Use inline expressions, for example: `msg: "{{ lookup('env', '{{ a_var }}') }}"` becomes `msg: "{{ lookup('env', a_var) }}"`

with the note that:

This feature will be disabled by default in ansible-core 2.23.