Consider adding `gh` to prow test-image

[dsimansk]

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

[dprotaso]

I'm good with whatever

/assign @dsimansk

[cardil]

Adding the following to the prow Dockerfile, should pre-cache the binary:

RUN go run github.com/cli/cli/v2/cmd/[email protected] --help > /dev/null