don't clobber existing RBENV_ROOT & follow Homebrew's default behavior

plugins/rbenv/rbenv.plugin.zsh

@@ -10,12 +10,18 @@ FOUND_RBENV=0
 rbenvdirs=("$HOME/.rbenv" "/usr/local/rbenv" "/opt/rbenv")
 if _homebrew-installed && _rbenv-from-homebrew-installed ; then
     rbenvdirs=($(brew --prefix rbenv) "${rbenvdirs[@]}")
+    if [[ $RBENV_ROOT = '' ]]; then 
+      RBENV_ROOT="$HOME/.rbenv"
+    fi
 fi
 
 for rbenvdir in "${rbenvdirs[@]}" ; do
   if [ -d $rbenvdir/bin -a $FOUND_RBENV -eq 0 ] ; then
     FOUND_RBENV=1
-    export RBENV_ROOT=$rbenvdir
+    if [[ $RBENV_ROOT = '' ]]; then
+      RBENV_ROOT=$rbenvdir
+    fi
+    export RBENV_ROOT
     export PATH=${rbenvdir}/bin:$PATH
     eval "$(rbenv init --no-rehash - zsh)"