Define Nix-enabled platform

Like bazelbuild/rules_docker, we define a Nix-enabled platform with
`@local_config_platform//:host` as the parent platform.

Deprecates `linux_x86_64_nixpkgs` and `darwin_x86_64_nixpkgs`.

.bazelrc

@@ -1 +1 @@
-build --host_platform=@io_tweag_rules_nixpkgs//nixpkgs/constraints:linux_x86_64_nixpkgs
+build --host_platform=@io_tweag_rules_nixpkgs//nixpkgs/platforms:host

nixpkgs/constraints/BUILD.bazel

@@ -20,6 +20,7 @@ platform(
         "@platforms//os:linux",
         "@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs",
     ],
+    deprecation = "Use @io_tweag_rules_nixpkgs//platforms:host instead.",
     visibility = ["//visibility:public"],
 )
 
@@ -30,5 +31,6 @@ platform(
         "@platforms//os:osx",
         "@io_tweag_rules_nixpkgs//nixpkgs/constraints:nixpkgs",
     ],
+    deprecation = "Use @io_tweag_rules_nixpkgs//platforms:host instead.",
     visibility = ["//visibility:public"],
 )

nixpkgs/platforms/BUILD.bazel

@@ -0,0 +1,6 @@
+platform(
+    name = "host",
+    constraint_values = ["@io_tweag_rules_nixpkgs//nixpkgs/constraints:support_nix"],
+    parents = ["@local_config_platform//:host"],
+    visibility = ["//visibility:public"],
+)