Don't use rustup.

Index: xtask/src/codegen.rs
--- xtask/src/codegen.rs.orig
+++ xtask/src/codegen.rs
@@ -130,7 +130,7 @@ impl fmt::Display for Location {
 fn reformat(text: String) -> String {
     let sh = Shell::new().unwrap();
     let rustfmt_toml = project_root().join("rustfmt.toml");
-    let version = cmd!(sh, "rustup run stable rustfmt --version").read().unwrap_or_default();
+    let version = cmd!(sh, "rustfmt --version").read().unwrap_or_default();
 
     // First try explicitly requesting the stable channel via rustup in case nightly is being used by default,
     // then plain rustfmt in case rustup isn't being used to manage the compiler (e.g. when using Nix).
@@ -150,7 +150,7 @@ fn reformat(text: String) -> String {
     } else {
         cmd!(
             sh,
-            "rustup run stable rustfmt --config-path {rustfmt_toml} --config fn_single_line=true"
+            "rustfmt --config-path {rustfmt_toml} --config fn_single_line=true"
         )
         .stdin(text)
         .read()
