basic/log: use getenv instead of secure_getenv

secure_getenv does not work when the process has a nonempty permitted
capability set, which means that it's unduly hard to configure logging in
systemd-logind, systemd-resolved, and others.

secure_getenv is useful for code in libraries which might get called from a
setuid application. log_parse_environment() is never called from our library
code, but directly form various top-level executables. None of them are
installed suid, and none are prepared to be used this way, since many
additional changes would be required to make that safe. We may just as well
drop the check and allow SYSTEMD_LOG_* to properly parsed.

Fixes #4900.