Replace #!/bin/env with #!/usr/bin/env in bot.py

Neither is POSIX, but /usr/bin/env is more widely used and /bin/env
does not exist on GNU Guix System.

bot.py

@@ -1,4 +1,4 @@
-#!/bin/env python3
+#!/usr/bin/env python3
 # SPDX-License-Identifier: MIT
 import asyncio, configparser
 from irctokens import build, Line