Use github tarball for krew installations

Krew tests need a plugin with a small tarball, like konfig. However, the krew install currently uses the manually created tarballs for installation, which is bad because the krew tests will skew the download statistics for konfig. This does not happen when using the github tarball.

Therefore: switch the krew installation method to the standard tarball and make sure that the help text is updated accordingly.

Bonus: keep unnecessary files out of the tarball (does GH support that?)