cni-flannel-plugin: 1.1 -> 1.0.0
cni-flannel-plugin: 1.1 -> 1.0.0

Update to a properly tagged version. Previously the tag was temporary,
and it was not obvious. See flannel-io/cni-plugin/issues/21

While here, also add some install time checking

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs