If it is not permissible to grow beard as per the Rules (bylaws) of India Air Force and you signed on the agreement in the beginning of your service then in this case you have only the two options, so if you are financially well off and you can find out any other income source comfortably after leaving this job then in such a case you may prefer quitting this job. But in case you cannot arrange for suitable income source then in such a case continue your present job and keep on doing taubah and istighfar. Also keep on looking for suitable job and when you get one leave this job.
And if as per the rules (bylaws) of Indian Air Force there is no restriction on growing beard rather you enjoy religious freedom of each type and there is no ban in adopting your religious customs as it is written in Indian Constitution, yet the department does not allow you to grow the beard then you should consult any expert advocate and materialize the religious freedom mentioned in the Indian Law.
Allah knows Best!
Darul Uloom Deoband