build: Pass $(NIX_INSTANTIATE) to the build and test environments.

* Makefile.am (.scm.go): Define $NIX_INSTANTIATE.
  (TESTS_ENVIRONMENT): Likewise, and define $NIX_HASH.
* guix/utils.scm (nixpkgs-derivation): Use $NIX_INSTANTIATE when
  defined.

* tests/utils.scm (%nix-hash): New variable.
  ("sha256 & bytevector->nix-base32-string"): Use it.