[SPARK-33727][K8S] Fall back from gnupg.net to openpgp.org

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

While building R docker image if we can't fetch the key from gnupg.net fall back to openpgp.org

### Why are the changes needed?

gnupg.net key servers are flaky and sometimes fail to resolve or return keys.

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

No

### How was this patch tested?

Tried to add key on my desktop, it failed, then tried to add key with openpgp.org and it succeed.

Closes #30696 from holdenk/SPARK-33727-gnupg-server-is-flaky.

Authored-by: Holden Karau <hkarau@apple.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/bindings/R/Dockerfile

@@ -29,7 +29,7 @@ RUN mkdir ${SPARK_HOME}/R
 RUN \
   echo "deb http://cloud.r-project.org/bin/linux/debian buster-cran35/" >> /etc/apt/sources.list && \
   apt install -y gnupg && \
-  apt-key adv --keyserver keys.gnupg.net --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF' && \
+  (apt-key adv --keyserver keys.gnupg.net --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF' || apt-key adv --keyserver keys.openpgp.org --recv-key 'E19F5F87128899B192B1A2C2AD5F960A256A04AF') && \
   apt-get update && \
   apt install -y -t buster-cran35 r-base r-base-dev && \
   rm -rf /var/cache/apt/*