Workaround for --override-input CHaP

[hamishmack]

This helps haskell.nix find the package source correctly. In particular new packages that are not in the main CHaP branch yet will be loaded correctly.

To try it out you will need a local CHaP, this can be build with:

git clone git@github.com:input-output-hk/cardano-haskell-packages.git
cd cardano-haskell-packages
nix develop -c foliage build -j 0 --write-metadata

That will make a cardano-haskell-packages/_repo dir with the CHaP repository including any new packages. To use from a directory containing this cardano-api branch run:

nix build --override-input CHaP path:/home/hamish/iohk/cardano-haskell-packages/_repo .\#hydraJobs.required

Make sure to use path: and the full path to the _repo directory.

[disassembler]

I added this commit to #76