Make a test more resilient against the host system

If the host system had $HOME/.cargo/config which configured a user name/email
then this test would fail because those would be prioritized over $USER.

Closes #514
auto merge of #631 : alexcrichton/cargo/issue-514, r=brson

If the host system had $HOME/.cargo/config which configured a user name/email
then this test would fail because those would be prioritized over $USER.

Closes #514