FEATURE: Add a command to update the UI tests

[shawntabrizi]

Something like bot fix ui, which will run the command here https://github.com/paritytech/substrate/blob/master/.maintain/update-rust-stable.sh

This should help prevent any stalled PRs where UI tests are failing.

This is not a really common issue, so not high priority, but hopefully it is as easy as I think it would be.

[mordamax]

@shawntabrizi is this only for substrate?
what would be for polkadot?

[altaua]

Implemented as bot update-ui $ <rust version> in paritytech/command-bot-scripts#37