Set up CI in rust-lang/rust for new bors

[jdno]

We are in the process of reimplementing bors in Rust. The implementation has progressed far enough that we want to start testing it on the rust-lang/rust repository. There are two possible approaches to make this happen:

1. Add support for the rust-lang-ci organization to the new bors implementation so that it can run tests there.
2. Configure the rust-lang/rust repository so that tests can be run there.

Historically, the rust-lang-ci organization was required to protect the secrets used in CI. But this isn't necessary anymore, and we want to eventually deprecate the organization anyways. So after some discussion in Zulip, we decided that we want to use the second approach for bors.

Tasks

This list is a work-in-progress.

• Create and configure the repository secrets that are required to run the test suite.