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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=h1) Report
> Merging [#25827](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e31b4f46df6ab1182c16183f48df76fb4beaca0f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25827/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25827      +/-   ##
==========================================
- Coverage   91.27%   91.27%   -0.01%     
==========================================
  Files         173      173              
  Lines       53002    53002              
==========================================
- Hits        48376    48375       -1     
- Misses       4626     4627       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25827/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=footer). Last update [e31b4f4...10954bd](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=h1) Report
> Merging [#25827](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e31b4f46df6ab1182c16183f48df76fb4beaca0f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25827/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25827      +/-   ##
==========================================
- Coverage   91.27%   91.27%   -0.01%     
==========================================
  Files         173      173              
  Lines       53002    53002              
==========================================
- Hits        48376    48375       -1     
- Misses       4626     4627       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25827/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=footer). Last update [e31b4f4...10954bd](https://codecov.io/gh/pandas-dev/pandas/pull/25827?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @qwhelan 