[services] Tunnelbroker - Append Rust build output folder to our global .gitignore

Summary:
Rust build output need to be added to our global .gitignore list to make the local builds ignores.
It was added near the same line for an Identity service.

Linear task: ENG-1330

Test Plan: Folder files are ignored in the git after running the Rust build using cargo build

Reviewers: varun, jonringer-comm, karol-bisztyga

Reviewed By: varun, jonringer-comm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4533