ci: add protoc on build

[iameli-streams]

So... this shouldn't be necessary. protoc is needed to build the files in net like net/lp_rpc.pb.go. Those files are committed to the repo; make shouldn't be rebuilding them. And yet, every so often, we get an error on build that looks like this:

protoc -I=. --go_out=plugins=grpc:. net/lp_rpc.proto
/bin/bash: line 1: protoc: command not found
make: *** [Makefile:7: net/lp_rpc.pb.go] Error 127
Error: Process completed with exit code 2.

I don't know why that happens. It doesn't happen once you rebuild. But... regenerating those files is harmless, I guess, so this will stop that for everybody forever. IDK.

[thomshutt]

@iameli shall we merge this?

[iameli]

@thomshutt Let me rebase, but I seem to recall this not solving the problem it was supposed to