surveyplan: use $(pwd) if $DESISURVEY isn't set about desisurvey

[sbailey]

Correction: surveyplan will exit with an informative message if $DESISURVEY isn't set. The problem was that our modules at NERSC use the BOSS-era standard of setting an environment variable that indicates the install location of the code. For desisurvey, that is, well $DESISURVEY. So I forgot to set $DESISURVEY and inherited the default, which points to a code directory.

We should consider deprecating these environment variables. Originally they were to help code find their associated config data, but with the exception of $DESIMODEL, that is no longer used (AFAIK). But that is a non-desisurvey issue. I'll close this.