What does this PR do?

Lock down flagfile, prevent global defaults, otherwise the osqueryi and osqueryd processes try to load defaults from
osquery.flags.defaults file from OSQUERY_HOME locations:
https://github.com/osquery/osquery/blob/2cd5b42c8f8fa52c6d251c6537595c7c59c90f4c/osquery/utils/config/default_paths.h

and can potentially mess with configuration if it was not specified by us via the command line flags.

This could be considered a bug or a feature, for example without this change you can create
/var/osquery/osquery.flags.defaults on Mac with content
--disable_tables=users
and this will prevent any osquery instance (including ours) from querying this table.

Why is it important?

This change prevents global osquery defaults flagfile from affecting our osquery instance configuration.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas

Screenshots

Empty osquery.flags file is created in the osquery "data" directory

Logs

Starting our osqueryd instance with flagfile parameter

{"log.level":"debug","@timestamp":"2021-08-26T10:27:57.567-0400","log.logger":"osquerybeat","log.origin":{"file.name":"osqd/osqueryd.go","file.line":157},"message":"start osqueryd process: args: [/Users/amaus/elastic/beats/x-pack/elastic-agent/build/distributions/elastic-agent-8.0.0-SNAPSHOT-darwin-x86_64/data/elastic-agent-8b4759/install/osquerybeat-8.0.0-SNAPSHOT-darwin-x86_64/osqueryd --force=true --disable_watchdog --utc --pidfile=osquery/osquery.pid --database_path=osquery/osquery.db --extensions_socket=/var/run/609582505/osquery.sock --logger_path=osquery --extensions_autoload=osquery/osquery.autoload --flagfile=osquery/osquery.flags --extensions_interval=3 --extensions_timeout=10 --config_plugin=osq_config --logger_plugin=osq_logger --config_refresh=60 --verbose --disable_logging=false]","service.name":"osquerybeat","ecs.version":"1.6.0"}