craterbot should generate unique names for each run

[varkor]

See rust-lang/rust#53578 (comment) for context. If the name parameter is not manually specified, it would be useful to disambiguate runs with pr-xxx-n where n is the index of the current crater run. This means that the user will not have to manually specify names for repeat crater runs.

[devonhollowood]

I'll take a shot at this.

[pietroalbini]

Thanks! If you have any doubts or questions feel free to ask a question here or ping me on the Rust Discord channel #crater.