Periodically remove beta artifacts from the collector

[Kobzol]

When benchmarking release (especially beta) artifacts, the collector accumulates Rust toolchains in $HOME/.rustup/toolchains. We should periodically clean them up, otherwise eventually it will run out of space.

@Mark-Simulacrum Do you remember if there was some cleanup script on the old machine before? Seems like this also had to be an issue there.

[Mark-Simulacrum]

I don't recall writing anything cleaning up old artifacts. I think we have a fairly big disk so we might have just not hit that point...