fix: correct environment check for devtool filename template

[chenjiahan]

Summary

Correct environment check for devtoolModuleFilenameTemplate. Using isDev (mode) to judge will be more accurate than using NODE_ENV.

• https://rspack.rs/config/output#outputdevtoolmodulefilenametemplate

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR replaces the previous NODE_ENV string check with the isDev flag for setting devtoolModuleFilenameTemplate, ensuring more accurate environment detection.

• switch environment check from env === 'development' to isDev
• remove unused env parameter from modifyBundlerChain callback
• update snapshot to include devtoolModuleFilenameTemplate

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/core/src/plugins/basic.ts	Use isDev instead of env for the source-map filename template
packages/core/tests/snapshots/default.test.ts.snap	Add devtoolModuleFilenameTemplate entry to the default snapshot

Comments suppressed due to low confidence (1)

packages/core/src/plugins/basic.ts:85

• Consider adding a test case for the production scenario (isDev = false) to verify that devtoolModuleFilenameTemplate is not set when not in development mode.

        if (isDev) {