fix(plugin-legacy): always fallback legacy build when CSP

[yoyo930021]

Description

Fixed #5919

Additional context

I use this answer to solve the problem of detecting dynamic imports needing eval.

Another answer also looks great, but we need a new file or allow data: in CSP, so I use the top answer.

You can use this project for test PR.
https://github.com/yoyo930021/vite-legacy-csp-bug

I test it in Edge 16, Edge 17, Edge 18, Chrome 62 with browserstack.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[visualjerk]

We experience the same issue. Thanks for this PR!

[DesselBane]

Same here.

[Jax-p]

looks good

[mrbar42]

@sodatea Any chance pushing this forward?
is help needed in any way?

[haoqunjiang]

I don't have the environment to test it.
But the code looks good to me.

[patak-dev]

LGTM too. Such a cool trick, thanks for the PR!