Override COLLECTOR_TAG at the builder stage

It's not necessary because the one isn't used there but feels
inconsistent otherwise. I'm torn about this commit.

collector/container/konflux.Dockerfile

@@ -57,6 +57,9 @@ ARG COLLECTOR_APPEND_CID=false
 ARG USE_VALGRIND=false
 ARG ADDRESS_SANITIZER=false
 ARG TRACE_SINSP_EVENTS=false
+ARG COLLECTOR_TAG
+
+ENV COLLECTOR_TAG="$COLLECTOR_TAG"
 
 WORKDIR ${BUILD_DIR}