fix(users/tazjin): rustfmt code with non-default settings

rustfmt only sometimes detects path-based nested config
files (probably some kind of race?), so my users folder uses a
separate formatting check for rustfmt to avoid flaky CI. Enough flakes
around already ...

Change-Id: Ifd862f9974f071b3a256643dd8e56c019116156a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5242
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/default.nix

@@ -0,0 +1,16 @@
+# //users/tazjin-specific CI configuration.
+{ pkgs, ... }:
+
+let
+  rustfmt = pkgs.writeShellScript "rustfmt-tazjin" ''
+    ${pkgs.fd}/bin/fd -e rs | \
+      ${pkgs.ripgrep}/bin/rg 'users/tazjin' | \
+      xargs ${pkgs.rustfmt}/bin/rustfmt --check --config-path users/tazjin
+  '';
+
+in
+rustfmt.overrideAttrs (_: {
+  meta.ci.extraSteps.rustfmt = {
+    command = rustfmt;
+  };
+})