The prospective employer did not rescind the offer because of your hair growth rate or your genetics. The offer was rescinded because an express condition was not timely satisfied. Big difference.
The drug test policy was in place at the time you applied; the policy applies to everyone -- all racial groups; the condition was articulated as a condition of the offer. Most likely the policy is not unlawful for its adverse impact on any racial group.
Realize that a candidate who wished to "beat" the drug test could covertly trim the hair during the "grow-in" phase, and the employer has the right to administer its race-neutral drug policy in ways that prevent that kind of covert conduct from succeeding.
It would be helpful to know why you had already cleft your prior job for a conditional job offer that was still conditional. That seems to have been imprudent and a knowing assumption of unnecessary risk on your part.
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