Use prebuilt protoc toolchain

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This downloads a prebuilt protoc binary and uses that through regular toolchain resolution (with the incompatible flag). This increases build speed and removes the many compiler warnings that pop-up during compilation of protoc sources. More importantly, works better on Windows where, for some reason, the protoc compilation fails from an external module:

gits/bb-deployments $ bazel build \
    --override_module protobuf=../protobuf \
    @protobuf//src/google/protobuf:timestamp_proto
ERROR: C:/tmp/eprbnvuo/external/protobuf~/src/google/protobuf/compiler/java/BUILD.bazel:99:11: Compiling src/google/protobuf/compiler/java/context.cc [for tool] failed: undeclared inclusion(s) in rule '@@protobuf~//src/google/protobuf/compiler/java:java':
this rule is missing dependency declarations for the following files included by 'src/google/protobuf/compiler/java/context.cc':
  'bazel-out/x64_windows-opt-exec-ST-d57f47055a04/bin/external/protobuf~/src/google/protobuf/compiler/java/_virtual_includes/java/google/protobuf/compiler/java/context.h'

Note that building protoc from its own checkout works:

~/gits/protobuf $ bazel build \
    //src/google/protobuf:timestamp_proto
Target //src/google/protobuf:timestamp_proto up-to-date:
  bazel-bin/src/google/protobuf/timestamp_proto-descriptor-set.proto.bin
INFO: Build completed successfully, 1 total action

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typo in commit message "downloades"

[moroten]

16 minutes instead of 24 minutes for the CI checks is a good improvement.

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typo in commit message "downloades"

Done