CI: exclude asv env directory from isort and tailing whitespace checks

[qwhelan]

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]

Codecov Report

Merging #25827 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	41.77% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	89.3% <0%> (-0.11%)	⬇️

---

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[codecov]

Codecov Report

Merging #25827 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	41.77% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	89.3% <0%> (-0.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e31b4f4...10954bd. Read the comment docs.

[WillAyd]

makes sense to me - thanks!

[jreback]

thanks @qwhelan