bugfix: support EDITOR values with spaces (#4523)
bugfix: support EDITOR values with spaces (#4523)

- previous code called `which` on $EDITOR, but that doesn't work for
  EDITORs like `emacs -nw` or `emacsclient -t -nw`.

- This patch just trusts EDITOR if it is set (same as previous
  behavior), and only uses the defaults if it's not.
LLNL/spack,TheTimmy/spack,iulian787/spack,tmerrick1/spack,TheTimmy/spack,lgarren/spack,LLNL/spack,EmreAtes/spack,krafczyk/spack,TheTimmy/spack,EmreAtes/spack,matthiasdiener/spack,matthiasdiener/spack,iulian787/spack,skosukhin/spack,tmerrick1/spack,mfherbst/spack,iulian787/spack,EmreAtes/spack,LLNL/spack,tmerrick1/spack,matthiasdiener/spack,EmreAtes/spack,mfherbst/spack,EmreAtes/spack,mfherbst/spack,matthiasdiener/spack,mfherbst/spack,skosukhin/spack,skosukhin/spack,TheTimmy/spack,LLNL/spack,mfherbst/spack,krafczyk/spack,iulian787/spack,lgarren/spack,lgarren/spack,lgarren/spack,tmerrick1/spack,skosukhin/spack,skosukhin/spack,TheTimmy/spack,krafczyk/spack,krafczyk/spack,lgarren/spack,matthiasdiener/spack,krafczyk/spack,tmerrick1/spack,iulian787/spack,LLNL/spack