Clean up Conan download cache

[jfpanisset]

When the script install_conanpackages.sh is called to install Conan packages, it leaves behind the Conan download cache in /tmp/downloads/c which includes the 1.6GB compressed archive for clang/llvm.

conan cache clean "*" --download

should allow the script to clean up the download folder.

[jfpanisset]

Can't seem to make conan cache clean work, just rm the files.

May or may no have to do with confusion between /opt/conan_home/.conan and .conan2 in Dockerfiles.