Switching "host" configurations to "exec" (#449)

diff --git a/bindgen/bindgen.bzl b/bindgen/bindgen.bzl
index 5e4db34..41603c8 100644
--- a/bindgen/bindgen.bzl
+++ b/bindgen/bindgen.bzl
@@ -197,27 +197,27 @@
         "bindgen": attr.label(
             doc = "The label of a `bindgen` executable.",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
         "rustfmt": attr.label(
             doc = "The label of a `rustfmt` executable. If this is provided, generated sources will be formatted.",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
             mandatory = False,
         ),
         "clang": attr.label(
             doc = "The label of a `clang` executable.",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
         "libclang": attr.label(
             doc = "A cc_library that provides bindgen's runtime dependency on libclang.",
-            cfg = "host",
+            cfg = "exec",
             providers = [CcInfo],
         ),
         "libstdcxx": attr.label(
             doc = "A cc_library that satisfies libclang's libstdc++ dependency.",
-            cfg = "host",
+            cfg = "exec",
             providers = [CcInfo],
         ),
     },
diff --git a/cargo/cargo_build_script.bzl b/cargo/cargo_build_script.bzl
index c8ea44e..9877422 100644
--- a/cargo/cargo_build_script.bzl
+++ b/cargo/cargo_build_script.bzl
@@ -147,7 +147,7 @@
             executable = True,
             allow_files = True,
             mandatory = True,
-            cfg = "host",
+            cfg = "exec",
         ),
         "crate_name": attr.string(
             doc = "Name of the crate associated with this build script target",
@@ -171,7 +171,7 @@
             executable = True,
             allow_files = True,
             default = Label("//cargo/cargo_build_script_runner:cargo_build_script_runner"),
-            cfg = "host",
+            cfg = "exec",
         ),
     },
     fragments = ["cpp"],
diff --git a/proto/proto.bzl b/proto/proto.bzl
index af90639..797bb74 100644
--- a/proto/proto.bzl
+++ b/proto/proto.bzl
@@ -293,7 +293,7 @@
         ),
         "_optional_output_wrapper": attr.label(
             executable = True,
-            cfg = "host",
+            cfg = "exec",
             default = Label(
                 "@io_bazel_rules_rust//proto:optional_output_wrapper",
             ),
@@ -372,7 +372,7 @@
         ),
         "_optional_output_wrapper": attr.label(
             executable = True,
-            cfg = "host",
+            cfg = "exec",
             default = Label(
                 "@io_bazel_rules_rust//proto:optional_output_wrapper",
             ),
diff --git a/proto/toolchain.bzl b/proto/toolchain.bzl
index 0d400a3..3cb3d3a 100644
--- a/proto/toolchain.bzl
+++ b/proto/toolchain.bzl
@@ -130,13 +130,13 @@
         "protoc": attr.label(
             doc = "The location of the `protoc` binary. It should be an executable target.",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
             default = Label("@com_google_protobuf//:protoc"),
         ),
         "proto_plugin": attr.label(
             doc = "The location of the Rust protobuf compiler plugin used to generate rust sources.",
             allow_single_file = True,
-            cfg = "host",
+            cfg = "exec",
             default = Label(
                 "@io_bazel_rules_rust//proto:protoc_gen_rust",
             ),
@@ -144,7 +144,7 @@
         "grpc_plugin": attr.label(
             doc = "The location of the Rust protobuf compiler plugin to generate rust gRPC stubs.",
             allow_single_file = True,
-            cfg = "host",
+            cfg = "exec",
             default = Label(
                 "@io_bazel_rules_rust//proto:protoc_gen_rust_grpc",
             ),
diff --git a/wasm_bindgen/wasm_bindgen.bzl b/wasm_bindgen/wasm_bindgen.bzl
index 15d8a35..14e9c81 100644
--- a/wasm_bindgen/wasm_bindgen.bzl
+++ b/wasm_bindgen/wasm_bindgen.bzl
@@ -109,7 +109,7 @@
         "bindgen": attr.label(
             doc = "The label of a `bindgen` executable.",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
     },
 )