common: install rustup into /opt/rustup

To avoid long-running chown's per https://github.com/dockcross/dockcross/issues/901

imagefiles/entrypoint.sh

@@ -32,13 +32,6 @@ if [[ -n $BUILDER_UID ]] && [[ -n $BUILDER_GID ]]; then
 
     export HOME=/home/${BUILDER_USER}
     shopt -s dotglob
-    # Move rustup/cargo directories as they are large, and not needed as root
-    if [[ -d /root/.rustup ]]; then
-        mv -t $HOME/ /root/.rustup
-    fi
-    if [[ -d /root/.cargo ]]; then
-        mv -t $HOME/ /root/.cargo
-    fi
     # Copy the rest
     cp -r /root/* $HOME/
     chown -R $BUILDER_UID:$BUILDER_GID $HOME