Ensure facts-spec that assumes CE uses a stub

What does this MR do?

Fixes a failing test when the spec tests run for a EE branch like https://gitlab.com/gitlab-org/omnibus-gitlab/-/pipelines/644670963/failures

Failures:
  1) Build::Facts.common_vars returns correct variables
     Failure/Error: expect(described_class.common_vars).to eq(result)
       expected: ["TOP_UPSTREAM_SOURCE_PROJECT=gitlab-org/gitlab", "TOP_UPSTREAM_SOURCE_REF=master", "TOP_UPSTREAM_SOU...BLIC_BUILDER_IMAGE_REGISTRY=registry.example.com", "COMPILE_ASSETS=false", "EDITION=CE", "ee=false"]
            got: ["TOP_UPSTREAM_SOURCE_PROJECT=gitlab-org/gitlab", "TOP_UPSTREAM_SOURCE_REF=master", "TOP_UPSTREAM_SOU...UBLIC_BUILDER_IMAGE_REGISTRY=registry.example.com", "COMPILE_ASSETS=false", "EDITION=EE", "ee=true"]
       (compared using ==)
     # ./spec/lib/gitlab/build/facts_spec.rb:76:in `block (3 levels) in <top (required)>'

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• N/A Test plan indicating conditions for success has been posted and passes
• N/A Documentation created/updated
• N/A Tests added
• N/A Integration tests added to GitLab QA
• N/A Equivalent MR/issue for the GitLab Chart opened