Fix build handling for chunkImportMap so it is scoped to the environment being built and works when builder.sharedPlugins is enabled. In a shared-plugin app, enabling chunk import maps for the client while disabling them for SSR must produce a complete client map, including extracted CSS references, without producing an SSR map. An environment that enables chunk import maps must emit its map even when emitAssets is false, preserve environment-specific options such as a custom output filename, and an environment that disables the feature must not retain an import-map asset merely because of its filename. Ensure client HTML and legacy-build processing follow the client environment's setting.