Don't use rustup.

Index: xtask/src/codegen.rs
--- xtask/src/codegen.rs.orig
+++ xtask/src/codegen.rs
@@ -146,7 +146,7 @@ impl fmt::Display for Location {
 }
 
 fn ensure_rustfmt(sh: &Shell) {
-    let version = cmd!(sh, "rustup run stable rustfmt --version").read().unwrap_or_default();
+    let version = cmd!(sh, "rustfmt --version").read().unwrap_or_default();
     if !version.contains("stable") {
         panic!(
             "Failed to run rustfmt from toolchain 'stable'. \
@@ -161,7 +161,7 @@ fn reformat(text: String) -> String {
     let rustfmt_toml = project_root().join("rustfmt.toml");
     let mut stdout = 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()
