Nix: read CABAL_IN_NIX_SHELL
Nix: read CABAL_IN_NIX_SHELL

cabal only cares if it is running in a Nix shell that was started by
another cabal process, so we should not read IN_NIX_SHELL; rather, we
read a cabal-specific variable for that purpose.

themoritz/cabal,themoritz/cabal,themoritz/cabal,mydaum/cabal,mydaum/cabal