makeBinaryWrapper: really unset NIX_CFLAGS

f8cc8ff575
fails to unset the variables in the environment due to a Bash quirk,
so set them to the empty string instead.

pkgs/build-support/setup-hooks/make-binary-wrapper.sh

@@ -33,8 +33,7 @@ assertExecutable() {
 # To troubleshoot a binary wrapper after you compiled it,
 # use the `strings` command or open the binary file in a text editor.
 makeWrapper() {
-    local NIX_CFLAGS_COMPILE NIX_CFLAGS_LINK
-    unset NIX_CFLAGS_COMPILE NIX_CFLAGS_LINK
+    local NIX_CFLAGS_COMPILE= NIX_CFLAGS_LINK=
     local original="$1"
     local wrapper="$2"
     shift 2