Name the preview variable in WSLENV

WSL shares only the variables listed there with a Windows vim.exe, as
s:preview_cmd already does for MSWINHOME. Untested, no WSL here.

bin/tagpreview.sh

@@ -35,7 +35,10 @@ fi
 # it cannot run a shell command or touch a file. ':sandbox' on its own only
 # 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}" \
+export FZFVIM_EXCMD="${EXCMD}"
+# WSL shares only the variables named in WSLENV with a Windows vim.exe
+export WSLENV="FZFVIM_EXCMD/w${WSLENV:+:${WSLENV}}"
+CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
               -c 'silent sandbox execute $FZFVIM_EXCMD' \
               -c 'let l=line(".") | new | put =l | print | qa!')" || exit