snmp: T3606: fix unknown notification OID
Commit 945300c6f5 ("snmp: mibs: import from vyatta-cfg-system") imported
the MIBs from vyatta-cfg-system and provided them with a symlink to
/usr/share/vyos/mibs. The symlink was never installed as the directory already
existed. Adjust the MIBDIRS environment variable to load the MIBs from the
proper VyOS folder and use no symlink at all.