refactor: register the pre-built protoc toolchain

Following discussions in buildbarn/bb-storage#248
we must register the toolchain as well. I don't know if this was always
an oversight or the behavior changed.

Author: stagnation

MODULE.bazel

@@ -29,7 +29,15 @@ bazel_dep(name = "rules_shell", version = "0.3.0")
 # but held back to be compatible with bb-remote-execution.
 bazel_dep(name = "rules_proto", version = "7.1.0")
 bazel_dep(name = "toolchains_llvm", version = "1.3.0")
-bazel_dep(name = "toolchains_protoc", version = "0.3.4")
+bazel_dep(name = "toolchains_protoc", version = "0.4.2")
+
+# Mirror the protoc setup from bb-storage: https://github.com/buildbarn/bb-storage/pull/248
+# NB: Match protobuf above, until it ships a pre-built binary toolchain itself
+protoc = use_extension("@toolchains_protoc//protoc:extensions.bzl", "protoc")
+protoc.toolchain(version = "v29.3")
+use_repo(protoc, "toolchains_protoc_hub")
+
+register_toolchains("@toolchains_protoc_hub//:all")
 
 # # Workarounds 2024-06-27
 # ERROR: in tag at https://bcr.bazel.build/modules/grpc-java/1.62.2/MODULE.bazel:89:15: no repository visible as '@com_google_protobuf_javalite' to the repository '@@grpc-java~', but referenced by label '@com_google_protobuf_javalite//:protobuf_javalite' in attribute 'target' of tag 'override'. Is the module 'grpc-java' missing a bazel_dep or use_repo(..., "com_google_protobuf_javalite")?