Don't use rustup.

Index: xtask/src/codegen.rs
--- xtask/src/codegen.rs.orig
+++ xtask/src/codegen.rs
@@ -131,27 +131,19 @@ fn reformat(text: String) -> String {
     let sh = Shell::new().unwrap();
     let rustfmt_toml = project_root().join("rustfmt.toml");
     let toolchain = &std::env::var("RUSTFMT_TOOLCHAIN").unwrap_or("stable".to_owned());
-    let version = cmd!(sh, "rustup run {toolchain} 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).
     let mut stdout = if !version.contains(toolchain) {
-        let version = cmd!(sh, "rustfmt --version").read().unwrap_or_default();
-        if !version.contains(toolchain) {
-            panic!(
-                "Failed to run rustfmt from toolchain '{toolchain}'. \
-                 Please run `rustup component add rustfmt --toolchain {toolchain}` to install it.",
-            );
-        } else {
-            cmd!(sh, "rustfmt --config-path {rustfmt_toml} --config fn_single_line=true")
-                .stdin(text)
-                .read()
-                .unwrap()
-        }
+        cmd!(sh, "rustfmt --config-path {rustfmt_toml} --config fn_single_line=true")
+            .stdin(text)
+            .read()
+            .unwrap()
     } else {
         cmd!(
             sh,
-            "rustup run {toolchain} rustfmt --config-path {rustfmt_toml} --config fn_single_line=true"
+            "rustfmt --config-path {rustfmt_toml} --config fn_single_line=true"
         )
         .stdin(text)
         .read()
