Build: set `GIT_SSH_COMMAND` to avoid host key checking (#12649)
<img width="677" height="106" alt="Screenshot_2025-12-10_17-36-31"
src="https://github.com/user-attachments/assets/6b977482-385f-4c4c-bd2f-0f294d68e4e4"
/>
Closes https://github.com/readthedocs/readthedocs-corporate/pull/2076
Closes https://github.com/readthedocs/readthedocs-corporate/issues/788