setup-hooks/strip.sh: use STRIP_FOR_TARGET, not TARGET_STRIP
setup-hooks/strip.sh: use STRIP_FOR_TARGET, not TARGET_STRIP

Since 1ac53985 "*-wrapper; Switch from `infixSalt` to `suffixSalt`"
(2020) 'TARGET_' prefix (and infix) is no more. '_FOR_TARGET' suffix
is the only used suffix for target-specific tools and flags.

Use that in stip instead of always-empty variable.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs