enable-coverage-instrumentation.sh: Fix unbound variable error

https://hydra.nixos.org/build/113299582

pkgs/build-support/setup-hooks/enable-coverage-instrumentation.sh

@@ -2,18 +2,7 @@ postPhases+=" cleanupBuildDir"
 
 # Force GCC to build with coverage instrumentation.  Also disable
 # optimisation, since it may confuse things.
-export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -O0 --coverage"
-
-# FIXME: Handle the case where postUnpack is already set.
-postUnpack() {
-    # This is an uberhack to prevent libtool from remoaving gcno
-    # files.  This has been fixed in libtool, but there are packages
-    # out there with old ltmain.sh scripts.  See
-    # http://www.mail-archive.com/libtool@gnu.org/msg10725.html
-    for i in $(find -name ltmain.sh); do
-        substituteInPlace $i --replace '*.$objext)' '*.$objext | *.gcno)'
-    done
-}
+export NIX_CFLAGS_COMPILE="${NIX_CFLAGS_COMPILE:-} -O0 --coverage"
 
 # Get rid of everything that isn't a gcno file or a C source file.
 # Also strip the `.tmp_' prefix from gcno files.  (The Linux kernel