While it would be nice to fix this bug in the general case, for the stricter case of supporting offline builds, I'm quite comfortable with punting that to the "user": as in, the user should provide a Python that has an upgraded pip in it before doing the build. For the online use case, I don't think anyone has ever noticed or cared.
Bug 1651669 Comment 3 Edit History
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While it would be nice to fix this bug in the general case, for the stricter case of supporting offline builds, I'm quite comfortable with punting that to the "user" as part of bug 1651662: as in, the user should provide a Python that has an upgraded pip in it before doing the build. For the online use case, I don't think anyone has ever noticed or cared.