Run the tag address in a sandbox in the preview too

A preview runs as soon as an entry is highlighted, so a crafted tags file did
not even need to be selected to get a shell command out of
'vim -c "silent {excmd}"'.

The preview Vim starts with '-u NONE' and exits right after, so the sandbox
alone is enough there; nothing it still permits outlives the process.

bin/tagpreview.sh

@@ -30,9 +30,12 @@ else
   exit 1
 fi
 
+# The address comes from a tags file, which can be untrusted, and a preview
+# runs as soon as an entry is highlighted. Sandboxed as a builtin tag jump is,
+# so that it cannot run a shell command or touch a file
 CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
-              -c "silent ${EXCMD}" \
+              -c "silent sandbox ${EXCMD}" \
               -c 'let l=line(".") | new | put =l | print | qa!')" || exit
 
 START_LINE="$(( CENTER - FZF_PREVIEW_LINES / 2 ))"