perf(build): enable unsafe fast drop in non-watch mode

[chenjiahan]

Summary

Enable Rspack's unsafe fast drop when running rspack build to improve build performance.

Before

After

Related Links

• perf: don't drop rust compiler instance in non-watch mode rspack#11920

Checklist

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

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[Copilot]

Pull Request Overview

This PR enables Rspack's unsafeFastDrop optimization in non-watch mode to improve build performance. The feature is controlled via an environment variable that's automatically set when running builds without watch mode.

Key changes:

• Sets RSPACK_UNSAFE_FAST_DROP environment variable to 'true' in non-watch build mode
• Reads the environment variable and enables unsafeFastDrop on the compiler instance

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/cli/commands.ts	Sets the environment variable when build is not in watch mode
packages/core/src/provider/createCompiler.ts	Reads environment variable and enables unsafe fast drop on the compiler

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