Fix a missing cfg=bzl flag that made all cpp kernel usage broken in OSS.

Enable //rust:all on the GHA CI, which would have caught this sooner.

PiperOrigin-RevId: 675990704

.github/workflows/test_rust.yml

@@ -29,7 +29,7 @@ jobs:
           bazel-cache: rust_linux
           bazel: >-
             test --cxxopt=-std=c++17 --host_cxxopt=-std=c++17
-            //rust:protobuf_upb_test //rust:protobuf_cpp_test
+            //rust:all
             //rust/test/rust_proto_library_unit_test:rust_upb_aspect_test
             //src/google/protobuf/compiler/rust/...
       - name: Run Cargo tests

rust/BUILD

@@ -130,7 +130,10 @@ rust_library(
     proc_macro_deps = [
         "@crate_index//:paste",
     ],
-    rustc_flags = ["--cfg=cpp_kernel"],
+    rustc_flags = [
+        "--cfg=cpp_kernel",
+        "--cfg=bzl",
+    ],
     deps = [
         ":utf8",
         "//rust/cpp_kernel:cpp_api",