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.

pkgs/build-support/setup-hooks/strip.sh

@@ -7,10 +7,10 @@ _doStrip() {
     # to $out anyways---if it does, that's a bigger problem that a lack of
     # stripping will help catch.
     local -ra flags=(dontStripHost dontStripTarget)
-    local -ra stripCmds=(STRIP TARGET_STRIP)
+    local -ra stripCmds=(STRIP STRIP_FOR_TARGET)
 
     # Optimization
-    if [[ "${STRIP-}" == "${TARGET_STRIP-}" ]]; then
+    if [[ "${STRIP-}" == "${STRIP_FOR_TARGET-}" ]]; then
         dontStripTarget+=1
     fi