helpers: fix non-atomic cache write

This was causing some values to be repeated more than once when using
multiple tmux windows at the same time, for example for the RAM
percentage in ram_percentage.sh.

GitHub: fixes #62

scripts/helpers.sh

@@ -130,8 +130,10 @@ put_cache_val() {
   val="${*:2}"
   tmpdir="$(get_tmp_dir)"
   [ ! -d "$tmpdir" ] && mkdir -p "$tmpdir" && chmod 0700 "$tmpdir"
-  get_time >"$tmpdir/$key"
-  echo -n "$val" >>"$tmpdir/$key"
+  (
+    get_time
+    echo -n "$val"
+  ) >"$tmpdir/$key"
   echo -n "$val"
 }