Fix the notification action for PRIVMSG in erc-notifications-notify

Fix the notification action for PRIVMSG in erc-notifications-notify

* lisp/erc/erc-desktop-notifications.el (erc-notifications-notify):
explicitly request the buffer for `nick', rather than relying on
(current-buffer) returning it.  That works fine for the very first
PRIVMSG sent by `nick', but ERC seems to handle subsequent PRIVMSGs
differently, where (current-buffer) would return the server buffer
rather than the existing buffer for PRIVMSGs from `nick'.