In reply to @nitely_ "you can also try": I'm currently working it with the
library maintainer.

1. checkout to the fork branch, nimble install fails
2. checkout to that branch in the rebased upstream, remove all but `bin` from
`~/.nimble`, and install and tests work
3. but checkout to the forked branch, install, then go back to upstream and it
fails.

...resolved by removing the nimbledps directory.