Using "tmpfs" as a script part for system.activationScripts is a bit
misleading since 6efcfe03ae4ef426b77a6827243433b5296613a4.
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>