merge cron.yml with ci.yml

[newAM]

Merges cron-ci.yml and ci.yml to keep cron-ci.yml up-to-date.

---

"Fixes" #83

This should only fail with new stabilized clippy lints or stabilized nightly features, but that has never happened.

This has only ever failed when it wasn't updated (#47, #83), or during network failures (#51)

[adamgreig]

I don't understand what caused the failure this week that wasn't caught by the normal PR CI? In principle this cron CI is to catch either a dependency or rust itself making a change that then only gets noticed when the next PR is opened, but if it's only flagging false positives I guess it's not justifying itself...

[newAM]

The cron-ci is compiling all examples on stable, and some examples now need nightly for the allocator_api feature, but the cron-ci wasn't updated when that was changed.

Alternatively I think(?) this could be folded into the main CI, with an if trigger == schedule condition to post an issue upon failure, then it would get updated with the main CI.

[adamgreig]

Ah, got it. Yea, if it's simple to have the main CI also run on schedule and only post a new issue on failure, maybe that's worth doing just in case something does start failing with one of the nightly features.

[newAM]

I think this should work:

      - uses: imjohnbo/issue-bot@v3
        if: |
          failure()
          && github.event_name == 'schedule'

I guess we will find out next week 😄