Fix docker file (rust-lang#2352)

Fix small issue introduced by rust-lang#2311

Author: celinval

scripts/ci/Dockerfile.bundle-release-20-04

@@ -25,5 +25,5 @@ RUN apt-get update && \
     rustup default $(rustup toolchain list | awk '{ print $1 }') && \
     cargo install --path ./kani-verifier && \
     cargo-kani setup --use-local-bundle ./kani-latest-x86_64-unknown-linux-gnu.tar.gz && \
-    apt-get clean
+    apt-get clean && \
     rm -rf /tmp/kani/* /root/.rustup/toolchains/*/share