kops: 1.8.0 -> 1.8.1

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin/bin/kops -h` got 0 exit code
- ran `/nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin/bin/kops --help` got 0 exit code
- ran `/nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin/bin/kops help` got 0 exit code
- ran `/nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin/bin/kops version` and found version 1.8.1
- found 1.8.1 with grep in /nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin
- found 1.8.1 in filename of file in /nix/store/hclwimgs404fz9k2lrx35xcxa16bgb9x-kops-1.8.1-bin

cc "@offline @zimbatm @ehmry @lethalman"