[SPARK-36198][TESTS] Skip UNIDOC generation in PySpark GHA job

### What changes were proposed in this pull request?
This PR aims to skip UNIDOC generation in PySpark GHA job.

### Why are the changes needed?

PySpark GHA jobs do not need to generate Java/Scala doc. This will save about 13 minutes in total.
-https://github.com/apache/spark/runs/3098268973?check_suite_focus=true
```
...
========================================================================
Building Unidoc API Documentation
========================================================================
[info] Building Spark unidoc using SBT with these arguments:  -Phadoop-3.2 -Phive-2.3 -Pscala-2.12 -Phive-thriftserver -Pmesos -Pdocker-integration-tests -Phive -Pkinesis-asl -Pspark-ganglia-lgpl -Pkubernetes -Phadoop-cloud -Pyarn unidoc
...
[info] Main Java API documentation successful.
[success] Total time: 192 s (03:12), completed Jul 18, 2021 6:08:40 PM
```

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Pass the GHA.

Closes #33407 from williamhyun/SKIP_UNIDOC.

Authored-by: William Hyun <william@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit c336f73ccd)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

.github/workflows/build_and_test.yml

@@ -170,6 +170,7 @@ jobs:
       HIVE_PROFILE: hive2.3
       GITHUB_PREV_SHA: ${{ github.event.before }}
       SPARK_LOCAL_IP: localhost
+      SKIP_UNIDOC: true
     steps:
     - name: Checkout Spark repository
       uses: actions/checkout@v2

dev/run-tests.py

@@ -397,7 +397,7 @@ def build_spark_assembly_sbt(extra_profiles, checkstyle=False):
     if checkstyle:
         run_java_style_checks(build_profiles)
 
-    if not os.environ.get("AMPLAB_JENKINS"):
+    if not os.environ.get("AMPLAB_JENKINS") and not os.environ.get("SKIP_UNIDOC"):
         build_spark_unidoc_sbt(extra_profiles)