Move toxiproxy.deb to /tmp (#326)

I am seeing Directory (/home/circleci/project) you are trying to checkout to is not empty and not a git repository error after I started using the new Dockerfile.ci image. My best guess is that this failure is because we download toxiproxy.deb file into the home directory which blocks git checkout.

This PR moves toxiproxy to /tmp/ to avoid this

Author: drdrsh

Dockerfile.ci

@@ -8,5 +8,5 @@ RUN sudo apt-get update && \
 	cargo install cargo-binutils rustfilt && \
 	rustup component add llvm-tools-preview && \
  	pip3 install psycopg2 && sudo gem install bundler && \
-	wget -O toxiproxy-2.4.0.deb https://github.com/Shopify/toxiproxy/releases/download/v2.4.0/toxiproxy_2.4.0_linux_$(dpkg --print-architecture).deb && \
-	sudo dpkg -i toxiproxy-2.4.0.deb
+	wget -O /tmp/toxiproxy-2.4.0.deb https://github.com/Shopify/toxiproxy/releases/download/v2.4.0/toxiproxy_2.4.0_linux_$(dpkg --print-architecture).deb && \
+	sudo dpkg -i /tmp/toxiproxy-2.4.0.deb