ci/nixpkgs-vet: fix random errors (#413239)

Changed files
+2
ci
+2
ci/nixpkgs-vet.nix
···
env.NIXPKGS_VET_NIX_PACKAGE = nix;
}
''
+
export NIX_STATE_DIR=$(mktemp -d)
+
nixpkgs-vet --base ${filtered base} ${filtered head}
touch $out