Set environment variables only on main threads

[luca3m]

This is done to reduce memory usage

[mstemm]

While you're adding checks for evt->m_tinfo being NULL, I think there are a few more parsing functions that need the check:

• sinsp_parser::parse_openat_dir
• sinsp_parser::parse_open_openat_creat_exit (there is an ASSERT, but an explicit exit in non debug builds would be useful)
• sinsp_parser::add_socket
• sinsp_parser::parse_close_exit
• sinsp_parser::parse_tracer (again, just an ASSERT)
• sinsp_parser::parse_rw_exit
• sinsp_parser::parse_fchdir_exit
• sinsp_parser::parse_getrlimit_setrlimit_exit

None of these are regressions, so I'm ok with filing an issue to track it and doing it later.

Otherwise, lgtm assuming tests pass!