nixos/activation: Rename "tmpfs" to "specialfs"

Using "tmpfs" as a script part for system.activationScripts is a bit
misleading since 6efcfe03

.

We no longer solely mount tmpfs within this script, so using "specialfs"
fits more nicely in terms of naming.

Tested against the "simple" NixOS installer test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>