gitoxide tests don't seem to be sandboxed

[ehuss]

Running the following command:

CARGO_PUBLIC_NETWORK_TESTS=1 __CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2=1 cargo test --test testsuite -- ssh

fails on my system because the ssh::bundled_github_works and ssh::invalid_github_key tests fail. The problem is that they are successfully connecting to github when the test expects them to fail. Something about our sandboxing is missing something that prevents gitoxide from grabbing ssh credentials from the user's environment.