tune nix deploy parameters (specifically for moby)

this is experimental; hard to understand immediately how significant are the effects

flake.nix

@@ -260,6 +260,10 @@
             nix build '.#nixosConfigurations.${host}.config.system.build.toplevel' --out-link ./result-${host} "$@"
             sudo nix sign-paths -r -k /run/secrets/nix_serve_privkey $(readlink ./result-${host})
 
+            # add more `-v` for more verbosity (up to 5).
+            # i copy the closure here separately from the nixos-rebuild mostly for the sake of introspectability.
+            nix-copy-closure -v --gzip --to "${host}" ./result-${host}
+
             # XXX: this triggers another config eval & (potentially) build.
             # if the config changed between these invocations, the above signatures might not apply to the deployed config.
             # let the user handle that edge case by re-running this whole command.

hosts/common/default.nix

@@ -47,6 +47,9 @@
     keep-outputs = true  # default: false
     # how many lines to show from failed build
     log-lines = 30  # default: 10
+    # how many substitution downloads to perform in parallel.
+    # i wonder if parallelism is causing moby's substitutions to fail?
+    max-substitution-jobs = 6  # default: 16
     # narinfo-cache-negative-ttl = 3600  # default: 3600
     # whether to use ~/.local/state/nix/profile instead of ~/.nix-profile, etc
     use-xdg-base-directories = true  # default: false