build: Update the builder image location. (#364)

Point Makefile to ghcr.io instead of DockerHub.

Makefile

@@ -1,4 +1,8 @@
-DOCKER_PROTO_BUILDER := docker run -v $(shell pwd):/workspace --workdir /workspace tendermintdev/docker-build-proto
+# The Docker image containing the generator, formatter, and linter.
+# This is generated by proto/Dockerfile. To update tools, make changes
+# there and run the Build & Push Proto Builder Image workflow.
+IMAGE := ghcr.io/tendermint/docker-build-proto:latest
+DOCKER_PROTO_BUILDER := docker run -v $(shell pwd):/workspace --workdir /workspace $(IMAGE)
 HTTPS_GIT := https://github.com/tendermint/spec.git
 
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