Keep @rules_cc//cc:toolchain_type as fully-qualified label always

Labels in toolchains attribute need to include the repository, otherwise the resolution won't work.

RELNOTES: None.
PiperOrigin-RevId: 279347045
Change-Id: Iba1eb8bb677771e2c089ab6bc1b5e019c4da434e
diff --git a/cc/private/rules_impl/compiler_flag.bzl b/cc/private/rules_impl/compiler_flag.bzl
index ccc9d45..4a59a03 100644
--- a/cc/private/rules_impl/compiler_flag.bzl
+++ b/cc/private/rules_impl/compiler_flag.bzl
@@ -25,5 +25,7 @@
     attrs = {
         "_cc_toolchain": attr.label(default = Label("//cc:current_cc_toolchain")),
     },
-    toolchains = ["//cc:toolchain_type"],
+    toolchains = [
+        "@rules_cc//cc:toolchain_type",  # copybara-use-repo-external-label
+    ],
 )