Add stack to the Nix shell

Otherwise rules_haskell will download a binary release which would not
work on NixOS.

```
DEBUG:
.../external/rules_haskell/haskell/cabal.bzl:2384:10:
Stack not found.
DEBUG:
.../external/rules_haskell/haskell/cabal.bzl:2385:10:
Downloading Stack 2.7.5 ...
```

shell.nix

@@ -16,6 +16,7 @@ mkShell {
     git
     openjdk11
     python3
+    stack
     # convenience dependencies
     less
   ];