Use rustup for nightly, so we can download from menteeth registry.

.cargo/config.toml

@@ -0,0 +1,8 @@
+[unstable]
+registry-auth = true
+
+[registries.menteeth]
+index = "ssh://git@git.shipyard.rs/menteeth/crate-index.git"
+
+[net]
+git-fetch-with-cli = true

flake.nix

@@ -37,16 +37,17 @@
           src = if inShell then null else ./.;
 
           buildInputs =
-            [ rustc
-              cargo
-              openssl
+            [ openssl
               pkg-config
             ] ++ (if inShell then [
               # In 'nix develop', provide some developer tools.
               rust-analyzer
               rustfmt
+              rustup
               clippy
             ] else [
+              rustc
+              cargo
               (import-cargo.builders.importCargo {
                 lockFile = ./Cargo.lock;
                 inherit pkgs;

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "nightly-2023-04-30"