GMR-157: do not set unchanged vars in xenv when generating shell scripts

apply Gaudi Merge Request 157: gaudi/Gaudi!157 (merged)

When using bash in conjunction with export -f, the string produced by xenv --sh cannot be used as argument to eval. Moreover, there is no real need to set variables if the value didn't change.

Note that the change affects only the options --sh and --csh of xenv.