Work around Bazel toolchain constraint issue

Toolchain resolution of the Python toolchain erroneously picked the
autodetecting toolchain instead of the nixpkgs provided toolchain.
Debugging with `--toolchain_resolution_debug` shows the following error:

```
Toolchain constraint @bazel_tools//tools/cpp:cc_compiler has value //nixpkgs/constraints:nixpkgs, which does not match value @io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs from the execution platform @io_tweag_rules_nixpkgs//nixpkgs/constraints:linux_x86_64_nixpkgs
```

The issue is that Bazel (tested at version 2.1) considers
`//nixpkgs/constraints:nixpkgs` and
`@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs` two different
values, even though they refer to the same target.

nixpkgs/constraints/BUILD.bazel

@@ -9,7 +9,7 @@ platform(
     constraint_values = [
         "@platforms//cpu:x86_64",
         "@platforms//os:linux",
-        ":nixpkgs",
+        "@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs",
     ],
     visibility = ["//visibility:public"],
 )
@@ -19,7 +19,7 @@ platform(
     constraint_values = [
         "@platforms//cpu:x86_64",
         "@platforms//os:osx",
-        ":nixpkgs",
+        "@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs",
     ],
     visibility = ["//visibility:public"],
 )