ansel: tell the build script we package ansel

We set BINARY_PACKAGE_BUILD=1
This removes the use of -march=native
and also compiles some low-level function
for different vector instruction sets

The upstream repo uses it in ci and packaging instructions.

pkgs/by-name/an/ansel/package.nix

···
       --prefix LD_LIBRARY_PATH ":" "$out/lib/ansel"
     )
   '';
+  cmakeFlags = [
+    "-DBINARY_PACKAGE_BUILD=1"
+  ];
 
   passthru.updateScript = unstableGitUpdater {
     # Tags inherited from Darktable, + a "nightly" 0.0.0 tag that new artefacts get attached to