During compilation, we build two derivations - one for the dependencies
and another for the user's crate itself. Passing `postInstall` to
`buildPackage` pass-throughs it into both derivations, which is not what
most users would probably expect.
This commit adds `postInstall` as an explicit configuration option (so
that it's neatly documented), zeroing it out for the dependencies-only
derivation.
I don't think there's any use case for something like `postInstallDeps`,
so it's not introduced here.
Closes https://github.com/nix-community/naersk/issues/237
