CI: fix coverage file location
CI: clean up some unused env variables

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=h1) Report
> Merging [#15792](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d3554c26aa85d66cbfe6f481464f1a357af5e12?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15792/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15792      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         143      143              
  Lines       49400    49400              
==========================================
- Hits        44963    44954       -9     
- Misses       4437     4446       +9
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15792?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/15792?src=pr&el=footer). Last update [9d3554c...8100d6d](https://codecov.io/gh/pandas-dev/pandas/pull/15792?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).