CI: exclude asv env directory from isort and tailing whitespace checks
When running `ci/code_checks.sh` on a local repository that has also been used to run `asv` benchmarks, a couple of the checks recurse into the `asv_bench/env` directory created for running benchmark environments. This massively increases the runtime and leads to linting errors on non-pandas packages, which is not informative.

This change simply adds `env` to the `isort` exclude list and the tailing whitespace `grep` call, which resolves the issue.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
