ghostscript: support SOURCE_DATE_EPOCH for reproducible builds

Without this patch, it is impossible to generate PDF documents in a reproducible way using Ghostscript, as CreationDate and ModDate are included in the PDF, which are based on the current time. I ran into this problem while using the LATEX command `\includegraphics` with an `.eps` graphic.
The Ghostscript developers are not interested in fixing this and rejected the patch, see https://bugs.ghostscript.com/show_bug.cgi?id=696765#c28.
[Debian carries the same patch](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=794004), as does [Arch Linux](https://gitlab.archlinux.org/archlinux/packaging/packages/ghostscript/-/commit/426e6bf6f290c1acf1816c032c630d921b626c95).

pkgs/by-name/gh/ghostscript/package.nix

@@ -2,6 +2,7 @@
 , stdenv
 , lib
 , fetchurl
+, fetchpatch2
 , pkg-config
 , zlib
 , expat
@@ -72,6 +73,12 @@ stdenv.mkDerivation rec {
   patches = [
     ./urw-font-files.patch
     ./doc-no-ref.diff
+
+    # Support SOURCE_DATE_EPOCH for reproducible builds
+    (fetchpatch2 {
+      url = "https://salsa.debian.org/debian/ghostscript/-/raw/01e895fea033cc35054d1b68010de9818fa4a8fc/debian/patches/2010_add_build_timestamp_setting.patch";
+      hash = "sha256-XTKkFKzMR2QpcS1YqoxzJnyuGk/l/Y2jdevsmbMtCXA=";
+    })
   ];
 
   outputs = [ "out" "man" "doc" "fonts" ];