nix.spec.in: Create /nix/store at install time.

Since we now chmod /nix/store at install time, we don't need to do it in the
post install script. We still chgrp in the post-install, because the nixbld
group doesn't exist at install time.

nix.spec.in

@@ -118,6 +118,10 @@ make DESTDIR=$RPM_BUILD_ROOT install
 
 find $RPM_BUILD_ROOT -name '*.la' -exec rm -f {} ';'
 
+# make the store
+mkdir -p $RPM_BUILD_ROOT/nix/store
+chmod 1775 $RPM_BUILD_ROOT/nix/store
+
 # make per-user directories
 for d in profiles gcroots;
 do
@@ -155,7 +159,6 @@ done
 
 %post
 chgrp %{nixbld_group} /nix/store
-chmod 1775 /nix/store
 %if ! 0%{?rhel}
 # Enable and start Nix worker
 systemctl enable nix-daemon.socket nix-daemon.service