[nix-local-build] Allow specifying freeze file for other commands

Right now the constraints file generated by `new-freeze` is called `cabal.project.freeze`, and the D.C.ProjectConfig code looks for it by that name. This makes it difficult to support multiple compiler versions in a project, though. The current workaround is to have CI symlink to the appropriate freeze file based on an environment variable, but this is pretty clunky.

Would it be possible to add a `--freeze-file=foo.freeze` option to the relevant commands? Would it make more sense to generalize to `--extra-config-file`, since these the freeze file is just mappended to the other config files?