ci breaks on branches in external repos

See https://github.com/MetaMask/metamask-module-template/actions/runs/3375550176/jobs/5602288539

It fetches from `github.com/MetaMask` and tries to check out the named branch regardless of actual origin.

Start with the linked GitHub Actions run and inspect the workflow step that fetches from MetaMask and checks out the named branch. Reproduce the failure using a branch in an external repository, then verify that the workflow uses the branch's actual origin and completes checkout successfully.

Written by the indexing model from the issue text.