better way of building protoc aarch64 artifacts (#26416)

tools/dockerfile/grpc_artifact_protoc_aarch64/Dockerfile

@@ -0,0 +1,16 @@
+# Copyright 2021 The gRPC Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# On aarch64 the protoc and codegen plugins are built via crosscompilation.
+FROM dockcross/linux-arm64

tools/run_tests/artifacts/artifact_targets.py

@@ -318,7 +318,7 @@ class ProtocArtifact:
                 if self.arch == 'aarch64':
                     # for aarch64, use a dockcross manylinux image that will
                     # give us both ready to use crosscompiler and sufficient backward compatibility
-                    dockerfile_dir = 'tools/dockerfile/grpc_artifact_python_manylinux2014_aarch64'
+                    dockerfile_dir = 'tools/dockerfile/grpc_artifact_protoc_aarch64'
                 environ['LDFLAGS'] += ' -static-libgcc -static-libstdc++ -s'
                 return create_docker_jobspec(
                     self.name,