[buildkite] Adding CI test for the Tunnelbroker client shared library

Summary:
This diff adds a Buildkite CI test for the Tunnelbroker client-shared library.
By running the cargo build and cargo test from the Nix in our CI we will make sure that the diffs changes are passing the CI test for the Tunnelbroker client library.

Linear task: ENG-2728

Test Plan: Running commands in the buildkite file from the project's root results in a Tunnelbroker client shared library successfully build.

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6282