[prebuilds] Disable prebuilds from bots by default

[AlexTugarev]

Considering push events caused by bots on GitHub for prebuild triggers doesn't seem to be a good idea:

• We register waves of push events potentially generating exceptional load.
• It's questionable that those prebuilds would be used a lot to start workspaces from.

The suggestion here is to flip the default and disable the prebuilds for such events. We can improve as a follow-up to introduce an option for projects to let them run on demand.

From the payload of a push event, we can easily detect if the change was caused by a bot:

    "sender": {
        "login": "dependabot[bot]",
        "id": 49699333,
        "avatar_url": "https://avatars.githubusercontent.com/in/29110?v=4",
        "url": "https://api.github.com/users/dependabot%5Bbot%5D",
        "html_url": "https://github.com/apps/dependabot",
        "type": "Bot",
        // ...
    },

[AlexTugarev]

cc. @atduarte and @geropl, I think this is low hanging and would increase efficiency.

[geropl]

Looking at this a bit more holistically:

• long term I'm sure we want a generic filter (regex?) for branches
• I could also imagine a generic filter (regex?) for logins (dependabot[bot])

So while I think a option on "type: Bot" (default: false) make sense on it's own, we might want to look beyond that to find sth that is stable long-term.

[stale[bot]]

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