shell.nix: use Haskell notation, use explicit attrs

Since variadic (...) attr passing does not work in Nix - the least of evils - is
to expicitly state the few attrs affecting the `shell.nix`.

Author: Anton-Latukha

shell.nix

@@ -1 +1,5 @@
-{} @ attrs: (import ./. attrs).env
+attrs@
+  { compiler ? "ghc8101"
+  , withHoogle ? true
+  }:
+(import ./. attrs).env