tests/nixos/s3-binary-cache-store: disable default substituter so it …

…runs faster

Since networking is disabled in these VMs, trying to talk to the default
cache.nixos.org slows the test down (since it can't resolve it).

(cherry picked from commit f0c1262)

tests/nixos/s3-binary-cache-store.nix

@@ -21,7 +21,10 @@ in {
         { virtualisation.writableStore = true;
           virtualisation.additionalPaths = [ pkgA ];
           environment.systemPackages = [ pkgs.minio-client ];
-          nix.extraOptions = "experimental-features = nix-command";
+          nix.extraOptions = ''
+            experimental-features = nix-command
+            substituters =
+          '';
           services.minio = {
             enable = true;
             region = "eu-west-1";
@@ -36,7 +39,10 @@ in {
       client =
         { config, pkgs, ... }:
         { virtualisation.writableStore = true;
-          nix.extraOptions = "experimental-features = nix-command";
+          nix.extraOptions = ''
+            experimental-features = nix-command
+            substituters =
+          '';
         };
     };