CI: split benchmarks from other checks
That job was the longest-running, so this should parallelize the various code checks with the benchmarks to get done in less wall clock time.

This pull request builds on https://github.com/pandas-dev/pandas/pull/39745.

cc @fangchenli

- [ ] ~~closes #xxxx~~
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
