mark cc_proto_library as manual

Author: thesayyn

examples/example_without_cc_toolchain/BUILD.bazel

@@ -9,4 +9,9 @@ proto_library(
 cc_proto_library(
     name = "cc_empty_proto",
     deps = [":empty_proto"],
+    # We already have a anaylsis test that asserts that the cc_proto_library
+    # uses the correct prebuilt toolchain, this cc_proto_library will never
+    # compile since the prebuilt toolchain is always behind the HEAD therefore
+    # will end up in version skew compilation errors.
+    tags = ["manual"],
 )