* fix(api): do not interpret "branch" placeholder when `GH_REPO` is set
Before, we would always interpret the "branch" placeholder, typically
setting it to the currently checked-out branch in the repository in the
current working directory. It didn't make sense to do that when the
`GH_REPO` environment variable was specified because the repository
would likely be different from the one in the current working directory.
Now, we instead report an error if both `GH_REPO` environment variable
and `branch` placeholder are specified.
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