protobuf: add version alignment hint

The version is important to match, as long as the default tensorflow
package we consume is a binary wheel that ships with a statically linked
protobuf version.

A mismatch of protobuf versions in a single process can lead to memory
corruption, because of ABI incompatibilities between protobuf versions.

pkgs/top-level/all-packages.nix

@@ -8240,6 +8240,8 @@ with pkgs;
 
   prospector = callPackage ../development/tools/prospector { };
 
+  # this version should align with the static protobuf version linked into python3.pkgs.tensorflow
+  # $ nix-shell -I nixpkgs=$(git rev-parse --show-toplevel) -p python3.pkgs.tensorflow --run "python3 -c 'import google.protobuf; print(google.protobuf.__version__)'"
   protobuf = protobuf_32;
 
   inherit