TST: Add pytest-instafail to environment.yml
Pretty useful pytest plugin for getting immediate feedback on test failures instead of waiting for an entire test suite to run:

```
(pandas-dev) ➜  pandas git:(add-pytest-instafail) ✗ pytest pandas/tests/groupby --instafail
============================================================================================ test session starts ============================================================================================
platform darwin -- Python 3.8.5, pytest-6.0.2, py-1.9.0, pluggy-0.13.1
rootdir: /Users/danielsaxton/pandas, configfile: setup.cfg
plugins: hypothesis-5.28.0, forked-1.2.0, asyncio-0.12.0, xdist-2.1.0, cov-2.10.1, instafail-0.4.1
collected 4533 items                                                                                                                                                                                        

pandas/tests/groupby/test_allowlist.py .............................................................................................................................................................. [  3%]
...........................................................                                                                                                                                           [  4%]
pandas/tests/groupby/test_apply.py ............x.x.x...................................................x..........................F

________________________________________________________________________________________________ test_fails _________________________________________________________________________________________________

    def test_fails():
>       assert False
E       assert False

pandas/tests/groupby/test_apply.py:1101: AssertionError

pandas/tests/groupby/test_apply_mutate.py ...                                                                                                                                                         [  6%]
pandas/tests/groupby/test_bin_groupby.py ..........                                                                                                                                                   [  7%]
pandas/tests/groupby/test_categorical.py .....................................................................................xxx..............................sss................................x.. [ 10%]
...
```
is this not exactly what
```
  -x, --exitfirst       exit instantly on first error or failed test.
```
does? part of pytest itself and we do not want to do this in CI builds. This continues running the test suite on test failure, only shows the output of failures. In general I think that's a bit more useful than exiting the whole run. ahh so it shows it as it goes, but still runs the entire thing, ok, that's useful. LGTM pending green thanks @dsaxton 