Don't redownload WASI SDK

[RReverser]

Description of Changes

The condition should be looking at the globally extracted WASI SDK, not at the temporary local archive.

This fixes an issue where WASI SDK would be redownloaded anew on new projects, even though it's supposed to be installed only once globally.

API and ABI breaking changes

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corresponding GitHub label.

Expected complexity level and risk

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but also to its interactions with existing and future code.

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Testing

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