common-aliases: lazily assign $BROWSER, $EDITOR and $XIVIEWER in ext aliases

Fixes #9212

plugins/common-aliases/common-aliases.plugin.zsh

@@ -55,15 +55,15 @@ if is-at-least 4.2.0; then
   # open browser on urls
   if [[ -n "$BROWSER" ]]; then
     _browser_fts=(htm html de org net com at cx nl se dk)
-    for ft in $_browser_fts; do alias -s $ft=$BROWSER; done
+    for ft in $_browser_fts; do alias -s $ft='$BROWSER'; done
   fi
 
   _editor_fts=(cpp cxx cc c hh h inl asc txt TXT tex)
-  for ft in $_editor_fts; do alias -s $ft=$EDITOR; done
+  for ft in $_editor_fts; do alias -s $ft='$EDITOR'; done
 
   if [[ -n "$XIVIEWER" ]]; then
     _image_fts=(jpg jpeg png gif mng tiff tif xpm)
-    for ft in $_image_fts; do alias -s $ft=$XIVIEWER; done
+    for ft in $_image_fts; do alias -s $ft='$XIVIEWER'; done
   fi
 
   _media_fts=(ape avi flv m4a mkv mov mp3 mpeg mpg ogg ogm rm wav webm)