fix(nix): use correct toolchain for rustfmt

nix/devshell.nix

@@ -9,7 +9,7 @@
     {
       # basic nix shell containing all things necessary to build continuwuity in all flavors manually (on x86_64-linux)
       devShells.default =
-        ((inputs.crane.mkLib pkgs).overrideToolchain (pkgs: self'.packages.stable-toolchain)).devShell
+        ((inputs.crane.mkLib pkgs).overrideToolchain (pkgs: self'.packages.dev-toolchain)).devShell
           {
             packages = [
               self'.packages.rocksdb

nix/rust.nix

@@ -27,9 +27,9 @@
           inherit stable-toolchain;
 
           dev-toolchain = fnx.combine [
-            stable-toolchain
             # use the nightly rustfmt because we use nightly features
             fnx.complete.rustfmt
+            stable-toolchain
           ];
         };
     };