ginac: build using gcc

On Darwin the stdenv uses clang, which builds binaries that crash with
example inputs, "SIGSEGV (Address boundary error)". Ginac documentation
says "We used GCC for development so if you have a different compiler
you are on your own." Switching to always using gcc aligns the
derivation with the build instructions.

pkgs/by-name/gi/ginac/package.nix

@@ -1,6 +1,7 @@
 {
   lib,
   stdenv,
+  gccStdenv,
   fetchurl,
   cln,
   pkg-config,
@@ -9,7 +10,7 @@
   python3,
 }:
 
-stdenv.mkDerivation (finalAttrs: {
+gccStdenv.mkDerivation (finalAttrs: {
   pname = "ginac";
   version = "1.8.10";