Add option for only using the main repo server

[lazka]

• In msys2-autobuild we depend on the state and the repo being in sync, so we only use the main repo there.
• When building msys2-installer we use the main mirror, just for safety (in theory it shouldn't make a difference, but it can't hurt to rule out external input).

We could add an experimental option to only use the main mirror. I wouldn't advertise it in the README though, it shouldn't be needed by users, would be slower, and would use up bandwidth of the main repo.