According to https://reviews.llvm.org/D96164, aarch64-darwin executables
require at least an ad hoc signature.
The build tool from the racket repo tries to sign $out/bin/racket but
errors out, because that binary already has a signature.
It is not clear yet at which stage the signature was introduced. This
patch removes the existing signature always before calling
add-ad-hoc-signature to circumvent that error.
