Regression: shells no longer contain `system-cxx-std-lib` even if necessary

[amesgen]

Since #2375, development shells (with exactDeps = false, the default) no longer contain system-cxx-std-lib, which means that pre-installed packages like double-conversion that depend on system-cxx-std-lib are broken and are not used by Cabal.

Concretely, running ghc-pkg check in a dev shell prints

The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
double-conversion-2.0.5.0

Minimal reproducible example

https://github.com/amesgen/stuff/tree/haskell.nix-shell-system-cxx-std-lib

da5e52c6 Include all pre-existing ghc pkgs in shell, unless exactDeps (#2375)
794d2e76 Update Hackage and Stackage

Running this command uses 794d2e7 (just before #2375) works fine (apart from printing some harmless warnings):

 $ nix develop github:amesgen/stuff/haskell.nix-shell-system-cxx-std-lib#good -c ghc-pkg check

Also:

 $ nix develop github:amesgen/stuff/haskell.nix-shell-system-cxx-std-lib#good -c ghc-pkg list | grep system-cxx-std-lib
    system-cxx-std-lib-1.0

OTOH, this command uses da5e52c (#2375) and prints the error message:

 $ nix develop github:amesgen/stuff/haskell.nix-shell-system-cxx-std-lib#bad -c ghc-pkg check
The following packages are broken, either because they have a problem
listed above, or because they depend on a broken package.
double-conversion-2.0.5.0

Also:

 $ nix develop github:amesgen/stuff/haskell.nix-shell-system-cxx-std-lib#bad -c ghc-pkg list | grep system-cxx-std-lib
WARNING: there are broken packages.  Run 'ghc-pkg-9.6.7 check' for more details.