`cabal build` doesn't find public sub package within development shell but builds fine with nix build.

[MangoIV]

When trying to cabal build a package that contains a public sublibrary, cabal fails to find this library within a shell provided by haskell.nix.

Somehow everything builds just fine with nix build.

Reproducer fails with:

Warning: No remote package servers have been specified. Usually you would have
one specified in the config file.
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: blup-0.1.0.0 (user goal)
[__1] next goal: bla (dependency of blup)
[__1] rejecting: bla-0.1.0.0/installed-Hsuvz2e1aJzCilZPkGBZy8 (does not
contain library 'blabla', which is required by blup)
[__1] fail (backjumping, conflict set: bla, blup)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: blup, bla