Force tests to use keras 2 (#6804)

This should resolve build breakages.

Author: groszewn

.github/workflows/ci.yml

@@ -101,7 +101,7 @@ jobs:
             grep -v 'external/com_google_protobuf/python: warning: directory' |&\
             grep -v 'INFO: From ProtoCompile ' )
       - name: 'Bazel: test (with TensorFlow support)'
-        run: bazel test //tensorboard/...
+        run: bazel test //tensorboard/... --test_env=TF_USE_LEGACY_KERAS=true
         if: matrix.tf_version_id != 'notf'
       - name: 'Bazel: test (non-TensorFlow only)'
         run: bazel test //tensorboard/... --test_tag_filters="support_notf"