Consider adding `gh` to prow test-image

As a follow-up to refactoring release script from `hub` to `gh` CLI tool.[1] We should consider installing the `gh` in Prow's test-image. It's a go based project so our `go_run` functions works well. But the release pace is a bit more frequent than for `hub`.
We could use debian's package to have at least almost latest version pre-installed.

/cc @dprotaso @upodroid @cardil

[1] https://github.com/knative/hack/pull/408/files#diff-8bc94a3006768345d17c827c0bb5840e6e4daee6de1c2b1ea672f53f162d171dR96