haskell-generic-builder: disable shared executable linking by default

This change raises the question of whether we should disable building of shared
libraries altogether (since we don't link them). Unfortunately, we can't,
because GHC expects shared libraries to be around when building and running
test suites, and no amount of passing --disable-shared flags at configure time
changes the fact. I *guess* that's related to the DYNAMIC_GHC_PROGRAMS = YES
flag we set when building GHC itself, but I haven't investigated it further
yet.

Closes https://github.com/NixOS/nixpkgs/issues/29011.