Utilize systemd OS information before falling back to distro-specific sources of information

[andrewpollock]

I see https://github.com/zcalusic/sysinfo/blob/85a453a29eca5b2812dd05007d78ce9522ad0eef/os.go#L32 uses the Linux distro-specific interfaces.

If systemd's /etc/os-release were consulted first, any Linux distro that uses systemd would be automatically detectable.