Don't overwrite EUPS_DIR in setups.*sh if already set at execution time

By setting EUPS_DIR before calling setups.*sh one can use a relocated EUPS installation.
Because EUPS_DIR may be initialised but not necessarily set in the environment. For instance, in `loadLSST.sh`we do:

    EUPS_DIR=$LSST_HOME/eups
    source $EUPS_DIR/bin/setups.sh

In this case, if EUPS_DIR is not set in the environment in `setups.sh`, `EUPS_DIR` would be available in the current shell but not in the sub-processes. Since in the current version of EUPS `setups.sh` you add `$EUPS_DIR` to the environment I assumed it needs to be setenv'ed.

The same answer holds for both `setups.sh` and `setups.csh`.