### What changes were proposed in this pull request?
Currently pip packaging test is being skipped:
```
========================================================================
Running PySpark packaging tests
========================================================================
Constructing virtual env for testing
Missing virtualenv & conda, skipping pip installability tests
Cleaning up temporary directory - /tmp/tmp.iILYWISPXW
```
See https://github.com/apache/spark/runs/2568923639?check_suite_focus=true
GitHub Actions's image has its default Conda installed at `/usr/share/miniconda` but seems like the image we're using for PySpark does not have it (which is legitimate).
This PR proposes to install Conda to use in pip packaging tests in GitHub Actions.
### Why are the changes needed?
To recover the test coverage.
### Does this PR introduce _any_ user-facing change?
No, dev-only.
### How was this patch tested?
It was tested in my fork: https://github.com/HyukjinKwon/spark/runs/2575126882?check_suite_focus=true
```
========================================================================
Running PySpark packaging tests
========================================================================
Constructing virtual env for testing
Using conda virtual environments
Testing pip installation with python 3.6
Using /tmp/tmp.qPjTenqfGn for virtualenv
Collecting package metadata (current_repodata.json): ...working... done
Solving environment: ...working... failed with repodata from current_repodata.json, will retry with next repodata source.
Collecting package metadata (repodata.json): ...working... done
Solving environment: ...working... done
## Package Plan ##
environment location: /tmp/tmp.qPjTenqfGn/3.6
added / updated specs:
- numpy
- pandas
- pip
- python=3.6
- setuptools
...
Successfully ran pip sanity check
```
Closes#32537 from HyukjinKwon/SPARK-35393.
Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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