Include shell.buildInputs in devshellFor

[zeme-wana]

Without shell.buildInputs, the final devshell may have an incomplete environment.
haskell.nix will successfully make a plan, but cabal build might fail, for example because it cannot find a package's sublibrary.

[michaelpj]

I commented on slack, but this doesn't seem like it can be the complete analysis. devshellFor has worked for ages, including for dependencies that have sub-libraries. So I can't see how this can be helping.

[angerman]

I tend to agree with michael here, this does not seem to be the right way to fix it.