nix: fix the rustfmt version by using nightly

Summary: Apparently this was broken. Maybe I broke it. Maybe something upstream
changed and caused a regression. But without it, we get the stable `rustfmt` in
the `nix develop` shell environment, not the nightly version. Fix it.

Signed-off-by: Austin Seipp <[email protected]>
Change-Id: I602ed8e5691c4d48f8db575d62624955

flake.nix

@@ -99,6 +99,12 @@
       formatter = pkgs.nixpkgs-fmt;
       devShells.default = pkgs.mkShell {
         buildInputs = with pkgs; [
+          # The CI checks against the latest nightly rustfmt, so we should too.
+          # NOTE (aseipp): include this FIRST before the rust-version override
+          # below; otherwise, it will be overridden by the rust-version and
+          # we'll get stable rustfmt instead.
+          (rust-bin.selectLatestNightlyWith (toolchain: toolchain.rustfmt))
+
           # Using the minimal profile with explicit "clippy" extension to avoid
           # two versions of rustfmt
           (rust-version.override {
@@ -108,9 +114,6 @@
             ];
           })
 
-          # The CI checks against the latest nightly rustfmt, so we should too.
-          (rust-bin.selectLatestNightlyWith (toolchain: toolchain.rustfmt))
-
           # Foreign dependencies
           openssl zstd libgit2 libssh2
           pkg-config