Fix Ginkgo v1 install command

[jochenehret]

What is this change about?

The current Ginkgo install command doesn't work anymore. This PR conflicts with https://github.com/cloudfoundry/cf-deployment-concourse-tasks/pull/127 (install Ginkgo v2). The cf-performance tests have not yet been migrated to v2, however.

Please provide contextual information.

The current install fails with:

# go install github.com/onsi/ginkgo/...
go: warning: "github.com/onsi/ginkgo/..." matched no packages

I would propose to replace this with:

# go install github.com/onsi/ginkgo/ginkgo@latest
go: downloading golang.org/x/tools v0.0.0-20201224043029-2b0845dc783e
go: downloading github.com/nxadm/tail v1.4.8
go: downloading golang.org/x/sys v0.0.0-20210112080510-489259a85091
go: downloading gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7
go: downloading github.com/fsnotify/fsnotify v1.4.9
# ginkgo version
Ginkgo Version 1.16.5

Please check all that apply for this PR:

• [ ] introduces a new task
• [ ] changes an existing task
• [x] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Fix for regression: Ginkgo v1 CLI is available again.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent