This updates the `release.nix` to now correctly run the test suite. You can
now build and test `grpc-haskell` by running:
```
$ nix-build -A grpc-haskell release.nix
```
... and `nix-shell` has been updated, too, so that you can do `cabal`
development inside of a `nix-shell` by running:
```
$ nix-shell -A grpc-haskell.env release.nix
```
For people who prefer to use `stack` you can still just build the `grpc`
library by running:
```
$ nix-build -A grpc release.nix
```
... then pass that library as input to `stack`
