Add cuda-maintainers cachix binary cache

upstream NixOS has a CUDA maintainers group now, so future seems to be brighter
for doing ML stuff in NixOS. The non-availability of binary caches was a problem
before but now it seems to be not so any more!

Add this cache for future-proofing this thing.

cachix/cuda-maintainers.nix

@@ -0,0 +1,11 @@
+
+{
+  nix.settings = {
+    substituters = [
+      "https://cuda-maintainers.cachix.org"
+    ];
+    trusted-public-keys = [
+      "cuda-maintainers.cachix.org-1:0dq3bujKpuEPMCX6U4WylrUDZ9JyUG0VpVZa7CNfq5E="
+    ];
+  };
+}