Bump rules_haskell to 0.13

We worked around the sub-package issue, so we can use a newer version.

This version brings a welcomed fix: better caching of the stack setup.
This was mentioned in the GitHub issue:
tweag/rules_haskell#1293. It seems like using
the `stack_snapshot_json` attribute might address all of the issues
rraised in that GitHub issue. If it only addresses the local
intermittent checks it would be a win by itself.

We'll try it out, and see where we get.

WORKSPACE.bazel

@@ -50,16 +50,10 @@ http_archive(
 
 http_archive(
     name = "rules_haskell",
-    patch_args = [
-        "-p1",
-    ],
-    patches = [
-        "//:package_root.patch",
-    ],
-    sha256 = "e1c4e460af15803cd779dd3234140b7d30b0eab669e239383a74c11d23e6be92",
-    strip_prefix = "rules_haskell-b9db31a285e697f98a2a9d4afebefb2aacc6b05a",
+    sha256 = "4d9e3a87ffada5e6417b7fe9ac84a80a7e817e3a044be57ca25da376edb2d0c7",
+    strip_prefix = "rules_haskell-0.13",
     urls = [
-        "https://github.com/tweag/rules_haskell/archive/b9db31a285e697f98a2a9d4afebefb2aacc6b05a.zip",
+        "https://github.com/tweag/rules_haskell/archive/v0.13.zip",
     ],
 )