use go version from builder image, instead of downloading in Dockerfile

[acornett21]

Description of the change:

• Updating the operator-sdk Dockerfile to copy go from the builder images, instead of downloading.

Motivation for the change:

• to speed up build time.
• to lessen burden of remembering to update the GO_VERSION for every update.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs