stdenv cc-wrapper: deal with edge-case regressions

Regression introduced in PR #81191 80729b6787.  The file does not exist
somewhere during bootstrap of pkgsStatic.busybox which is used in nix
(by default).

I tested the builds.

pkgs/build-support/cc-wrapper/default.nix

@@ -366,7 +366,9 @@ stdenv.mkDerivation {
     # There are a few tools (to name one libstdcxx5) which do not work
     # well with multi line flags, so make the flags single line again
     + ''
-      substituteInPlace $out/nix-support/libc-cflags --replace $'\n' ' '
+      if [ -e "$out/nix-support/libc-cflags" ]; then
+        substituteInPlace "$out/nix-support/libc-cflags" --replace $'\n' ' '
+      fi
 
       substituteAll ${./add-flags.sh} $out/nix-support/add-flags.sh
       substituteAll ${./add-hardening.sh} $out/nix-support/add-hardening.sh