build: make protobuf optional in depends

Those that want to build it can now pass PROTOBUF=1.

ci/test/00_setup_env_i686.sh

@@ -7,6 +7,7 @@
 export LC_ALL=C.UTF-8
 
 export HOST=i686-pc-linux-gnu
+export DEP_OPTS="PROTOBUF=1"
 export PACKAGES="g++-multilib python3-zmq"
 export GOAL="install"
 export BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-bip70 --enable-glibc-back-compat --enable-reduce-exports LDFLAGS=-static-libstdc++"