nixos/gnome/at-spi2-core: fix disabling a11y in all contexts
`environment.variables` gets sourced by shells & the x11 wrapper
through bash's `/etc/profile`, but not by systemd services, dbus
services, wayland sessions...
`environment.sessionVariables` sets these variables with PAM early in
the login process so it gets applied in all contexts.
I ran into a similar issue before in #109060.