[SPARK-33565][INFRA][FOLLOW-UP][3.0] Keep the test coverage with Pyth…

…on 3.8 in GitHub Actions

### What changes were proposed in this pull request?

This is a backport PR of #30510

This PR proposes to keep the test coverage with Python 3.8 in GitHub Actions. It is not tested for now in Jenkins due to an env issue.

**Before this change in GitHub Actions:**

```
========================================================================
Running PySpark tests
========================================================================
Running PySpark tests. Output is in /__w/spark/spark/python/unit-tests.log
Will test against the following Python executables: ['/usr/bin/python3', 'python2.7', 'pypy3']
...
```

**After this change in GitHub Actions:**

```

========================================================================
Running PySpark tests
========================================================================
Running PySpark tests. Output is in /__w/spark/spark/python/unit-tests.log
Will test against the following Python executables: ['python3.8', 'python2.7', 'pypy3']
```

### Why are the changes needed?

To keep the test coverage with Python 3.8 in GitHub Actions.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

GitHub Actions in this build will test.

Closes #30511 from HyukjinKwon/SPARK-33565-3.0.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

dev/run-tests.py

@@ -484,6 +484,12 @@ def run_python_tests(test_modules, parallelism, with_coverage=False):
     if test_modules != [modules.root]:
         command.append("--modules=%s" % ','.join(m.name for m in test_modules))
     command.append("--parallelism=%i" % parallelism)
+    if "GITHUB_ACTIONS" in os.environ:
+        # See SPARK-33565. Python 3.8 was temporarily removed as its default Python executables
+        # to test because of Jenkins environment issue. Once Jenkins has Python 3.8 to test,
+        # we should remove this change back and add python3.8 into python/run-tests.py script.
+        command.append("--python-executable=%s" % ','.join(
+            x for x in ["python3.8", "python2.7", "pypy3", "pypy"] if which(x)))
     run_cmd(command)
 
     if with_coverage: