nixos/pulseaudio: disable flat-volumes by default

The motivation for this is that some applications are unaware
of this feature and can set their volume to 100% on startup
harming people ears and possiblly blowing someone's audio
setup.

I noticed this in #54594 and by extension epiphany[0].

Please also note that many other distros have this default for
the reason outlined above.

Closes #5632 #54594

[0]: https://bugzilla.gnome.org/show_bug.cgi?id=675217