nixos/tests/blivet: Fix by avoiding "nix-store".

The "nix-store" command within the VM test is running without
NIX_REMOTE=daemon and since Nix 1.8 tries to open the store database in
read-write mode even for nix-store -qR.

Now, we're doing this properly and rely on setup hooks, which is the
same method that's used when you're building a library which depends on
blivet.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 2af435b5cd74140bf8ee6c476793ee6ed7c1cba4)