fix: always read targets options (fix: vitejs#15788)

If `modernPolyfills` is set to `true`, but `renderLegacyChunks` is set
to false, `genLegacy` will be false and we would return early. If this
return happens before `modernTargets` is initialized, the modern
polyfills will be generated with the wrong (default?) target instead of
the specified `modernTargets`.

packages/plugin-legacy/src/index.ts

@@ -325,10 +325,6 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
       }
       config = _config
 
-      if (!genLegacy || config.build.ssr) {
-        return
-      }
-
       targets =
         options.targets ||
         browserslistLoadConfig({ path: config.root }) ||
@@ -339,6 +335,10 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
       isDebug &&
         console.log(`[@vitejs/plugin-legacy] modernTargets:`, modernTargets)
 
+      if (!genLegacy || config.build.ssr) {
+        return
+      }
+
       const getLegacyOutputFileName = (
         fileNames:
           | string