tidy: in ci, use npm ci.

Author: lolbinarycat

src/tools/tidy/src/ext_tool_checks/rustdoc_js.rs

@@ -5,6 +5,7 @@ use std::ffi::OsStr;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+use build_helper::ci::CiEnv;
 use ignore::DirEntry;
 
 use crate::walk::walk_no_read;
@@ -14,15 +15,21 @@ pub(super) fn npm_install() -> Result<(), super::Error> {
     // disable a bunch of things we don't want.
     // this makes tidy output less noisy, and also significantly improves runtime
     // of repeated tidy invokations.
-    let mut child = Command::new("npm")
-        .args(&["install", "--audit=false", "--save=false", "--fund=false"])
-        .spawn()?;
+    let mut cmd = Command::new("npm");
+    if CiEnv::is_ci() {
+        // `npm ci` redownloads every time and thus is too slow for local development.
+        cmd.arg("ci");
+    } else {
+        cmd.arg("install");
+    }
+    cmd.args(&["--audit=false", "--save=false", "--fund=false"]);
+    let mut child = cmd.spawn()?;
     match child.wait() {
         Ok(exit_status) => {
             if exit_status.success() {
                 return Ok(());
             }
-            Err(super::Error::FailedCheck("npm install failed"))
+            Err(super::Error::FailedCheck("npm install"))
         }
         Err(error) => Err(super::Error::Generic(format!("npm install failed: {error:?}"))),
     }