cabal exec will sometimes output other messages to stdout, which
broke the build. It used to be intermittent in CI, now seems to always
happen. Messages are eg "Resolving dependencies..."

It seems that cabal list-bin never does this. I hope.

cabal list-bin is fairly new, needing cabal 3.8, which is only in Debian
testing/unstable. So fall back to cabal exec if it fails.