triagebot.toml caching is troublesome

[ehuss]

triagebot.toml is cached fairly aggressively. This can cause difficulty, particularly during development. When updating the file, it can take a long time for that update to be reflected.

There are two aspects to this:

1. triagebot caches the file for at least two minutes. This cannot be overridden. The only way to stop this is to restart the server.
2. GitHub's https://raw.githubusercontent.com/ endpoint seems to cache for several minutes, too. I do not understand how that caching works, or if there is some header we need to set to fix that. In my investigation, I found odd behavior where triagebot would receive the old content, but running curl on the command-line would get the new content, which was very confusing.

Some random thoughts:

• Can triagebot invalidate its own internal cache when it gets a webhook indicating that triagebot.toml has been updated? I believe we can get this list of files from the PushEvent commit list (there are limits, but shouldn't be an issue here).
• Are there headers we can include to https://raw.githubusercontent.com/ to bust its cache? Or is there an alternate endpoint for getting the raw file that doesn't have this behavior?

[Kobzol]

FWIW, in new bors I use GitHub's API to download the contents of the bors config file from the default branch (https://github.com/rust-lang/bors/blob/main/src/github/api/client.rs#L56). That hopefully shouldn't be cached at all, since it reads git state directly.

Other than that, I also reload the config only periodically though.