ci/nixpkgs-vet: try to fix race

This is another attempt at fixing the annoying nixpkgs-vet errors in CI,
which just throw with `error: SQLite database '...' is busy`.

The assumption is that this happens while initially setting up the state
directories. nixpkgs-vet runs `nix-instantiate` on both the base and the
head commit and these two could interfere.

ci/nixpkgs-vet.nix

@@ -32,6 +32,7 @@ runCommand "nixpkgs-vet"
   }
   ''
     export NIX_STATE_DIR=$(mktemp -d)
+    $NIXPKGS_VET_NIX_PACKAGE/bin/nix-store --init
 
     nixpkgs-vet --base ${filteredBase} ${filteredHead}