Enable requiring a particular workflow-cli version/commit

Currently for testing PRs in this repo, we check out the master branch of the `workflow-cli` repo and build it from scratch with the controller-sdk-go changes.

We should create a mechanism for checking out a forked `workflow-cli` repo at a certain commit if it is required by, for instance, `Requires workflow-cli#`.

Inspect how this repository currently tests pull requests, checks out the master branch of workflow-cli, and builds it with controller-sdk-go changes. Trace where a requirement such as `Requires workflow-cli#` could be read and determine how a fork and commit should be selected. Done means testing can use the required workflow-cli revision while existing tests continue to work.

Written by the indexing model from the issue text.