zsh: unset empty TMP and TMPDIR

this is a safety mechanism so that `/` is never used

nix/home-manager/programs/zsh.nix

@@ -53,6 +53,15 @@ in
         inNixShell = ''$([[ -n "$IN_NIX_SHELL" ]] && printf " 🐚")'';
       in
       ''
+        if test ! -n "$TMPDIR" -a -z "$TMPDIR"; then
+          unset TMPDIR
+        fi
+
+        if test ! -n "$TMP" -a -z "$TMP"; then
+          unset TMP
+        fi
+
+
         PROMPT='%F{%(!.red.green)}%n%f@%m %(?.%F{green}✓%f.%F{red}✗ ($?%))%f %F{blue}%~%f${inNixShell}%F{magenta}$(git_prompt_info)%f$prompt_newline%_%F{%(!.red.green)}$(prompt_char)%f '
         RPROMPT=""