modules/nixos/buildbot: disable topic, switch to repoAllowlist

[zowoq]

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[zowoq]

#1598 (comment)
Isn't the github topic less work overall than having this list synchronously managed?

@Mic92

Responding here as the other PR was rebased on this one.

For users in nix-community, yes, it is more work as now they either need to send a PR or ask me to do it.

For users outside nix-community, no, as they only need to add the repo to the list, they don't need to bother with adding the topic.

For admins, no, the list is less work and simpler. Buildbot usually needs to be restarted manually anyway to pick up topic changes and it also allows me to enable/disable repos temporarily or permanently without needing to make changes to the repos, adding/removing topics or unarchiving/archiving.

We could revisit this in the future but at the moment this is preferable.