Deny setting CARGO_HOME in `[env]` table

[ehuss]

After some discussion (and tying into #10094 and #11589), we should consider denying setting CARGO_HOME in [env] now. Cargo itself does not change its home directory based on that value. Recursive calls to cargo will use that new value. We suspect that the likelyhood of someone already doing this is low, and we'd like to close off the possibility sooner rather than later. We think it is the wrong behavior to only pass it to recursive invocations, we'd like more control over this, particularly until we decide the semantics of #11589.