Run CI on more operating systems other than `ubuntu-latest`

Currently, we run our CI only on `ubuntu-latest`. If we would like to support different operating systems, we should run our CI on them too. GitHub actions natively support MacOS and Windows so enabling them would be a start. The benefit of this is 1) more users 2) [capturing issues related to filesystem early](https://github.com/google/oss-rebuild/pull/714#discussion_r2298442929).