Keep the whole tag address inside the preview sandbox

':sandbox' is a modifier and covers only the command up to the first '|', so an
address like '/pat/|!touch file' still ran the shell command as soon as an entry
was highlighted. ':execute' on a value taken from the environment keeps all of
it inside the sandbox, and takes the address out of the '-c' string as well.

:Helptags previews a plugin's doc/tags through the same script.

bin/tagpreview.sh

@@ -30,12 +30,14 @@ else
   exit 1
 fi
 
-# The address comes from a tags file, which can be untrusted, and a preview
+# The address comes from a tags file, which can be untrusted, and a preview runs
-# runs as soon as an entry is highlighted. Sandboxed as a builtin tag jump is,
+# as soon as an entry is highlighted. Sandboxed as a builtin tag jump is, so that
-# so that it cannot run a shell command or touch a file
+# it cannot run a shell command or touch a file. ':sandbox' on its own only
-CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
+# covers the command up to the first '|', hence 'sandbox execute' on a value
+# passed through the environment, which keeps the whole address inside
+CENTER="$(FZFVIM_EXCMD="${EXCMD}" "${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
-              -c "silent sandbox ${EXCMD}" \
+              -c 'silent sandbox execute $FZFVIM_EXCMD' \
               -c 'let l=line(".") | new | put =l | print | qa!')" || exit
 
 START_LINE="$(( CENTER - FZF_PREVIEW_LINES / 2 ))"