Upgrade rules_nixpkgs to 0.11.1

rules_haskell 0.19 depends on rules_nixpkgs 0.10, but we need the fix
tweag/rules_nixpkgs#421 to avoid error:

```
panic: rulesGoStdlibPrefix should have been set via -X
```

Also, load only the needed toolchain rules from rules_nixpkgs.

WORKSPACE.bazel

@@ -16,6 +16,13 @@ load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")
 
 bazel_skylib_workspace()
 
+http_archive(
+    name = "io_tweag_rules_nixpkgs",
+    sha256 = "2a555348d7f8593fca2bf3fc6ce53c5d62929de81b6c292e23f16c557c0ae45a",
+    strip_prefix = "rules_nixpkgs-0.11.1",
+    urls = ["https://github.com/tweag/rules_nixpkgs/releases/download/v0.11.1/rules_nixpkgs-0.11.1.tar.gz"],
+)
+
 # Download rules_haskell and make it accessible as "@rules_haskell".
 http_archive(
     name = "rules_haskell",
@@ -40,6 +47,21 @@ load("@rules_sh//sh:posix.bzl", "sh_posix_configure")
 # Bring in a "local" toolchain - (only hermetic if in a pure nix-shell!)
 sh_posix_configure()
 
+# Add rules_nixpkgs toolchains
+load(
+    "@io_tweag_rules_nixpkgs//nixpkgs:repositories.bzl",
+    "rules_nixpkgs_dependencies",
+)
+
+rules_nixpkgs_dependencies(
+    toolchains = [
+        "cc",
+        "go",
+        "python",
+        "posix",
+    ],
+)
+
 load(
     "@rules_haskell//haskell:repositories.bzl",
     "rules_haskell_dependencies",
@@ -51,9 +73,15 @@ rules_haskell_dependencies()
 # Load nixpkgs_git_repository from rules_nixpkgs,
 # which was already initialized by rules_haskell_dependencies above.
 load(
-    "@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl",
+    "@rules_nixpkgs_cc//:cc.bzl",
     "nixpkgs_cc_configure",
+)
+load(
+    "@rules_nixpkgs_core//:nixpkgs.bzl",
     "nixpkgs_local_repository",
+)
+load(
+    "@rules_nixpkgs_python//:python.bzl",
     "nixpkgs_python_configure",
 )
 

flake.nix

@@ -78,7 +78,7 @@
             nativeBuildInputs = nativeBuildInputs ++ [ ghc ];
 
             fetchAttrs = {
-              sha256 = "sha256-ZyP1umO8wqcwEaP5tYCtGDGfCztK3naZmmrWx/rGBao=";
+              sha256 = "sha256-u2bC9SUJEJd4RCrnZhIRKtLcyesyxe94hnwsBI8+3s4=";
             };
 
             buildAttrs = {