Fix exe to allow overriding

[cidkidnix]

Make it easier to override exe to provide a modified backend, frontend, etc..

I have:

• [x] Based work on latest develop branch
• [ ] Followed the contribution guide
• [ ] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [ ] Run the test suite: $(nix-build -A selftest --no-out-link)
• [ ] Updated the changelog
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)