Prevent duplicate migrations_paths

[#50099107]

If Msf::DBManager#initialize_metasploit_data_models is run multiple
times, such as during specs, ActiveRecord::Migrator.migrations_paths was
getting populated with multiple copies of the metasploit_data_models
db/migrate path, which would lead to 'DB.migrate threw an exception:
Multiple migrations have the version number 0' errors in framework.log.