feat(gs/chupacabra): Use mugwump as a binary cache

Since buildkite is running on there, it'll be nice to be able to
download things. Obviously if this laptop ever becomes a laptop again
this'll have to go away (or just become the external domain)

Change-Id: I5fc49c061dbf79f8d523244bcf822e8d96fa6d42
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2047
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/system/system/machines/chupacabra.nix

@@ -114,4 +114,7 @@
 
   # Necessary to get steam working
   hardware.opengl.driSupport32Bit = true;
+
+  nix.binaryCaches = [ "ssh://grfn@172.16.0.5" ];
+  nix.trustedBinaryCaches = [ "ssh://grfn@172.16.0.5" ];
 }