haskellSrc2nix: disallow substitutes

Fix #49818. See the bug for more details.

The work done by haskellSrc2nix is usually lightweight, but needs to
be done each time a cabal file change. This triggers unexpected
network activity for a cache query which results most of the time on a
cache miss.

By disabling substitutes we:

- avoid the, mostly useless, binary cache query
- avoid unexpected network failure when users change a cabal file and
  try to rebuild without connectivity.