perf(@angular-devkit/build-angular): with esbuild builder only load Sass compiler when needed

Previously, the Sass compiler was imported on the start of every build regardless of its usage.
The Sass compiler will now only be loaded if a Sass stylesheet is requested.

(cherry picked from commit 87a61aa)

Author: clydin

packages/angular_devkit/build_angular/src/builders/browser-esbuild/sass-plugin.ts

@@ -16,12 +16,10 @@ export function createSassPlugin(options: { sourcemap: boolean; loadPaths?: stri
     setup(build: PluginBuild): void {
       let sass: typeof import('sass');
 
-      build.onStart(async () => {
-        // Lazily load Sass
-        sass = await import('sass');
-      });
+      build.onLoad({ filter: /\.s[ac]ss$/ }, async (args) => {
+        // Lazily load Sass when a Sass file is found
+        sass ??= await import('sass');
 
-      build.onLoad({ filter: /\.s[ac]ss$/ }, (args) => {
         try {
           const warnings: PartialMessage[] = [];
           // Use sync version as async version is slower.