pyenv: Detect $PYENV_ROOT and act accordingly.

This also fixes issues for Homebrew users if the followed the caveat to
set PYENV_ROOT to use the homebrew created directories.

plugins/pyenv/pyenv.load

-if test -d $HOME/.pyenv/bin
-  set PATH $HOME/.pyenv/bin $PATH
-end
-
-if test -d $HOME/.pyenv/shims
-  set PATH $HOME/.pyenv/shims $PATH
+if test -n "$PYENV_ROOT"
+  if test -d $PYENV_ROOT/bin
+    set PATH $PYENV_ROOT/bin $PATH
+  end
+  if test -d $PYENV_ROOT/shims
+    set $PATH $PYENV_ROOT/shims $PATH
+  end
+else
+  if test -d $HOME/.pyenv/bin
+    set PATH $HOME/.pyenv/bin $PATH
+  end
+  if test -d $HOME/.pyenv/shims
+    set PATH $HOME/.pyenv/shims $PATH
+  end
 end