dist/common/systemd: set WorkingDirectory to get heap dump correctly
dist/common/systemd: set WorkingDirectory to get heap dump correctly

Currently scylla-jmx.service's PWD is "/", we get following error when
JVM trying to write heap dump on current directory:

Aug 17 05:52:15 localhost.localdomain scylla-jmx[3469]: Starting the JMX server
Aug 17 05:52:16 localhost.localdomain scylla-jmx[3469]: java.lang.OutOfMemoryError: Java heap space
Aug 17 05:52:16 localhost.localdomain scylla-jmx[3469]: Dumping heap to java_pid3469.hprof ...
Aug 17 05:52:16 localhost.localdomain scylla-jmx[3469]: Unable to create java_pid3469.hprof: Permission denied

To fix this, we need to specify WorkingDirectory on systemd unit.

scylladb/scylla-jmx,scylladb/scylla-jmx,scylladb/scylla-jmx