Prevent accidentally getting dependencies with cabal

[alexfmpe]

This make sure all dependencies are provided by nix and cabal can't won't download any, preventing one from forgetting to re-launch the shell after adding a not-currently-available package.

For instance, when doing cabal build all inside nix-shell -A shells.ghcjs

I have:

• 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)

[ymeister]

I have concerns regarding how this will play out with haskell.nix branch which is expected to become default somewhere in the future. haskell.nix uses cabal.project and *.cabal files to get the build plan, so setting active-repositories might interfere with that (though it needs to be checked).