tcb: fix cross-compilation of tcb

Without the change build fails to find `gcc`:

    $ nix build --no-link -f. pkgsCross.riscv64.shadow
    > make[1]: Entering directory '/build/source/libs'
    > gcc -O2  -I../include -Wall -Wextra -fPIC -c libtcb.c -o libtcb.o
    > /nix/store/b9jxsncywlygxjjyql0z5bq6z29m6j4r-bash-5.1-p16/bin/bash: line 1: gcc: command not found

pkgs/tools/security/tcb/default.nix

@@ -26,6 +26,9 @@ stdenv.mkDerivation rec {
       --replace "PREFIX = /usr" "PREFIX = $out" \
       --replace "SBINDIR = /sbin" "SBINDIR = $bin/bin" \
       --replace "INCLUDEDIR = \$(PREFIX)/include" "INCLUDEDIR = $dev/include"
+
+    # Override default 'CC=gcc'
+    makeFlagsArray+=("CC=$CC")
   '';
 
   meta = with lib; {