Add plugin_format_flag to j2objc_proto_toolchain in bazel_tools.

tools/j2objc/BUILD file is split into BUILD and BUILD.tools, where the latter ends up in @bazel_tools repository.

The split was necessary because new attribute is not available in the Bazel we bootstrap Bazel@HEAD with.

The split up also removed dependency on @bazel_j2objc repository.

PiperOrigin-RevId: 413071660
diff --git a/src/create_embedded_tools.py b/src/create_embedded_tools.py
index df5f8e3..4621ca7 100644
--- a/src/create_embedded_tools.py
+++ b/src/create_embedded_tools.py
@@ -31,6 +31,7 @@
     ('*tools/jdk/BUILD.tools', lambda x: 'tools/jdk/BUILD'),
     ('*tools/build_defs/repo/BUILD.repo',
      lambda x: 'tools/build_defs/repo/BUILD'),
+    ('*tools/j2objc/BUILD.tools', lambda x: 'tools/j2objc/BUILD'),
     ('*tools/platforms/BUILD.tools', lambda x: 'platforms/BUILD'),
     ('*tools/platforms/*', lambda x: 'platforms/' + os.path.basename(x)),
     ('*tools/cpp/BUILD.tools', lambda x: 'tools/cpp/BUILD'),