igraph: Only build and test x86_64-linux.

aarch64-linux tests fail on the community builder, so don't build there
until we have more evidence that failing tests are not a problem.

pkgs/development/libraries/igraph/default.nix

@@ -30,6 +30,8 @@ stdenv.mkDerivation rec {
     description = "The network analysis package";
     homepage = "https://igraph.org/";
     license = lib.licenses.gpl2;
+    # NB: Known to fail tests on aarch64.
+    platforms = [ "x86_64-linux" ];
     maintainers = [ lib.maintainers.MostAwesomeDude ];
   };
 }