Previously, `isAnalyzingDefaultBranch` was failing because there are
some missing env vars: `GITHUB_SHA`, `GITHUB_REF`, and
`GITHUB_EVENT_PATH`. Also, `checkout_path` is missing as an input.
Rather than trying to set them to mock values, which would require
setting the paths to existing paths in the file system, I chose to stub
the entire function. I think this is fine since the point of the test
is to check the ram and threads values, not testing the
`isAnalyzingDefaultBranch` function.
The GitHub API client coerces `fake-server-url` to the Dotcom API URL,
which means commands like `util.getGitHubVersion` will call the Dotcom
API with the `fake-token`, resulting in 401s.
We therefore use the Dotcom URL instead and additionally stub
`util.getGitHubVersion` as a good practice (it's no longer necessary).
