Don't use `.cargo` for custom CARGO_HOME

Don't use `.cargo` for custom CARGO_HOME

Users changing cargo's home directory expect it to be used as top path
instead of there being a `.cargo` hidden dir in it. There are some cases
following this pattern one of them is python's virtualenvwrapper, which
uses `WORK_HOME` as a home dir for everything.

This patch removes the extra `.cargo` and assumes that CARGO_HOME is the
real path a user wants to use as a homedir for cargo.