[SPARK-32233][TESTS] Disable SBT unidoc generation testing in Jenkins

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

This PR aims to disable SBT `unidoc` generation testing in Jenkins environment because it's flaky in Jenkins environment and not used for the official documentation generation. Also, GitHub Action has the correct test coverage for the official documentation generation.

- https://github.com/apache/spark/pull/28848#issuecomment-654577911 (amp-jenkins-worker-06)
- https://github.com/apache/spark/pull/28926#issuecomment-654316537 (amp-jenkins-worker-06)
- https://github.com/apache/spark/pull/28969#issuecomment-654918636 (amp-jenkins-worker-06)
- https://github.com/apache/spark/pull/28975#issuecomment-654447955 (amp-jenkins-worker-05)
- https://github.com/apache/spark/pull/28986#issuecomment-654416543  (amp-jenkins-worker-05)
- https://github.com/apache/spark/pull/28992#issuecomment-654371469 (amp-jenkins-worker-06)
- https://github.com/apache/spark/pull/28993#issuecomment-655289237 (amp-jenkins-worker-05)
- https://github.com/apache/spark/pull/28999#issuecomment-653976760 (amp-jenkins-worker-04)
- https://github.com/apache/spark/pull/29010#issuecomment-655246083 (amp-jenkins-worker-03)
- https://github.com/apache/spark/pull/29013#issuecomment-654292483 (amp-jenkins-worker-04)
- https://github.com/apache/spark/pull/29016#issuecomment-654495070 (amp-jenkins-worker-05)
- https://github.com/apache/spark/pull/29025#issuecomment-654889938 (amp-jenkins-worker-04)
- https://github.com/apache/spark/pull/29042#issuecomment-655587989 (amp-jenkins-worker-03)

### Why are the changes needed?

Apache Spark `release-build.sh` generates the official document by using the following command.
- https://github.com/apache/spark/blob/master/dev/create-release/release-build.sh#L341

```bash
PRODUCTION=1 RELEASE_VERSION="$SPARK_VERSION" jekyll build
```

And, this is executed by the following `unidoc` command for Scala/Java API doc.
- https://github.com/apache/spark/blob/master/docs/_plugins/copy_api_dirs.rb#L30

```ruby
system("build/sbt -Pkinesis-asl clean compile unidoc") || raise("Unidoc generation failed")
```

However, the PR builder disabled `Jekyll build` and instead has a different test coverage.
```python
# determine if docs were changed and if we're inside the amplab environment
# note - the below commented out until *all* Jenkins workers can get `jekyll` installed
# if "DOCS" in changed_modules and test_env == "amplab_jenkins":
#    build_spark_documentation()
```

```
Building Unidoc API Documentation
========================================================================
[info] Building Spark unidoc using SBT with these arguments:
-Phadoop-3.2 -Phive-2.3 -Pspark-ganglia-lgpl -Pkubernetes -Pmesos
-Phadoop-cloud -Phive -Phive-thriftserver -Pkinesis-asl -Pyarn unidoc
```

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

No. (This is used only for testing and not used in the official doc generation.)

### How was this patch tested?

Pass the Jenkins without doc generation invocation.

Closes #29017 from dongjoon-hyun/SPARK-DOC-GEN.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

dev/run-tests.py

@@ -363,7 +363,8 @@ def build_spark_assembly_sbt(extra_profiles, checkstyle=False):
     if checkstyle:
         run_java_style_checks(build_profiles)
 
-    build_spark_unidoc_sbt(extra_profiles)
+    if not os.environ.get("AMPLAB_JENKINS"):
+        build_spark_unidoc_sbt(extra_profiles)
 
 
 def build_apache_spark(build_tool, extra_profiles):