[perl #112652] [STAR] make rakudo-spectest tries to use git

# New Ticket Created by  Will Coleda 
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... but in star, t/spec is no longer a git checkout.

The make command continues, but we should skip the git command if
t/spec isn't a checkout.

• [perl #112652] [STAR] make rakudo-spectest tries to use git by Will Coleda