[FLINK-5814] [build] Fix packaging flink-dist in unclean source direc…

…tory

If "<flink-dir>/build-target" already existed, running 'mvn package' for
flink-dist would create a symbolic link inside "<flink-dir>/build-target"
instead of replacing that symlink. This commit fixes this behaviour of 'ln -sf'
by adding the --no-dereference parameter.

This closes apache#3331

flink-dist/pom.xml

@@ -275,7 +275,7 @@ under the License.
 								<configuration>
 									<executable>ln</executable>
 									<arguments>
-										<argument>-sf</argument>
+										<argument>-sfn</argument>
 										<argument>${project.basedir}/target/flink-${project.version}-bin/flink-${project.version}</argument>
 										<argument>${project.basedir}/../build-target</argument>
 									</arguments>