L2SS-290: Add a check to the Makefile that the lofar20rc.sh file has been sourced
If the LOFAR20_DIR environment variable is not set, then bail out with an error message. This reminds the user to source the lofar20rc.sh file. It also avoids weird error messages when trying to run Docker containers without the necessary mount options set up in our environment variables.
Closes L2SS-290