fix: always read targets options (fix: #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`.

Also, config.build.target must be set from the `modernTargets`, even if
`genLegacy` is to set.

packages/plugin-legacy/src/index.ts

@@ -204,15 +204,13 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
           config.build.cssTarget = 'chrome61'
         }
 
-        if (genLegacy) {
-          // Vite's default target browsers are **not** the same.
-          // See https://github.com/vitejs/vite/pull/10052#issuecomment-1242076461
-          overriddenBuildTarget = config.build.target !== undefined
-          overriddenDefaultModernTargets = options.modernTargets !== undefined
-          config.build.target = options.modernTargets
-            ? resolveToEsbuildTarget(browserslist(options.modernTargets))
-            : modernTargetsEsbuild
-        }
+        // Vite's default target browsers are **not** the same.
+        // See https://github.com/vitejs/vite/pull/10052#issuecomment-1242076461
+        overriddenBuildTarget = config.build.target !== undefined
+        overriddenDefaultModernTargets = options.modernTargets !== undefined
+        config.build.target = options.modernTargets
+          ? resolveToEsbuildTarget(browserslist(options.modernTargets))
+          : modernTargetsEsbuild
       }
 
       return {
@@ -325,10 +323,6 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
       }
       config = _config
 
-      if (!genLegacy || config.build.ssr) {
-        return
-      }
-
       targets =
         options.targets ||
         browserslistLoadConfig({ path: config.root }) ||
@@ -339,6 +333,10 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
       isDebug &&
         console.log(`[@vitejs/plugin-legacy] modernTargets:`, modernTargets)
 
+      if (!genLegacy || config.build.ssr) {
+        return
+      }
+
       const getLegacyOutputFileName = (
         fileNames:
           | string