Add missing yum clean all to JAVA install block in AL2 (ARM) (#485)

This fixes an issue with the PowerShell Core installation, as an empty
continuation line at the end of the JAVA install block was messing with
ENV POWERSHELL_VERSION being set. Adds the `&& yum clean all` present in
the AL2 x86_64 image.

Author: jpculp

al2/aarch64/standard/2.0/Dockerfile

@@ -203,6 +203,7 @@ ENV PATH "/usr/local/bin/sbt/bin:$PATH"
 RUN sbt version
 # Cleanup
 RUN rm -fr /tmp/* /var/tmp/* \
+    && yum clean all
 #****************     END JAVA     ****************************************************