org-agenda-show-current-time-in-grid: Use more common Unicode arrow

* lisp/org-agenda.el (org-agenda-current-time-string): Use more
commonly available "←" char indicator instead of "⭠".  Suggested in
https://orgmode.org/list/875yk1e3h1.fsf@posteo.net.  This patch does
not fully solve the problem with `char-displayable-p' returning
non-nil when the char cannot be actually displayed in agenda, but it
should at least make the issue less severe until we find a more
accurate test.  In any case, the Unicode version of the agenda grid
will be available for more users with this patch.

lisp/org-agenda.el

@@ -1587,9 +1587,9 @@ will align with agenda items."
 
 (defcustom org-agenda-current-time-string
   (if (and (display-graphic-p)
-           (char-displayable-p ?⭠)
+           (char-displayable-p ?←)
            (char-displayable-p ?─))
-      "⭠ now ───────────────────────────────────────────────"
+      "← now ───────────────────────────────────────────────"
     "now - - - - - - - - - - - - - - - - - - - - - - - - -")
   "The string for the current time marker in the agenda."
   :group 'org-agenda-time-grid