sslscan: fix clang build

/cc ZHF #36454

(cherry picked from commit 0fdbf3edeb7e58d174aaa9dc881adee471174827)

pkgs/tools/security/sslscan/default.nix

···
 
   buildInputs = [ openssl ];
 
-  installFlags = [ "PREFIX=$(out)" ];
+  makeFlags = [ "PREFIX=$(out)" "CC=cc" ];
 
   meta = with stdenv.lib; {
     description = "Tests SSL/TLS services and discover supported cipher suites";