nixos/offlineimap: Don't build the package on eval

Coercing the derivation to string causes the package to be built during
evaluation rather than during actual realization which is completely
unnecessary because we don't need additional Nix expression information
for the package (nor do we need it for the service).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @DamienCassou
Cc: @Profpatsch (stumbled on this because of him)