Revert changes to docker file from rust-lang#2311 (rust-lang#2354)

It looks like the other changes done by rust-lang#2311 to the release docker file were not as harmless as I thought. This change reverts them to fix the release action.

Author: celinval

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

@@ -8,8 +8,6 @@ ENV DEBIAN_FRONTEND=noninteractive \
     DEBCONF_NONINTERACTIVE_SEEN=true \
     PATH="/root/.cargo/bin:${PATH}"
 WORKDIR /tmp/kani
-COPY ./tests ./tests
-COPY  ./Cargo.toml ./Cargo.toml
 COPY ./kani-*-x86_64-unknown-linux-gnu.tar.gz ./kani-latest-x86_64-unknown-linux-gnu.tar.gz
 # Very awkward glob (not regex!) to get `kani-verifier-*` and not `kani-verifier-*.crate`
 COPY ./target/package/kani-verifier-*[^e] ./kani-verifier