Automatic end-date calculation fails during promotion window

[ehuss]

If the --end date is not specified, then cargo-bisect-rustc will assume the end date is today. However, there is a window of an hour or so from UTC midnight until nightly gets published that this will result in an error:

ERROR: Could not find nightly-2023-01-20; url: https://static.rust-lang.org/dist/2023-01-20/rustc-nightly-x86_64-apple-darwin.tar.gz

I think it would be good if when --end is not specified that cargo-bisect-rustc would walk back from "today" at least a few days to find a nightly that exists. There is already code for it to skip over missing nightlies while bisecting. I'm not sure if there is something that can be reused, or if there are other options.

cargo-bisect-rustc 0.6.5