Should only happen when something is doing open(..., "w") on the Setup binary while it's running. I don't see why Cabal itself should do that (without external concurrency).

Extra suspicious is that there is an empty line at line 1765 in that file.
So far I found that errors in such stdenv-linux/setup lines are always bogus, somebody should find out why that is, or what program prints that line ... output, because it's apparently wrong.