fix(deps): update dependency @netlify/build to ^15.6.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@netlify/build	^15.5.0 -> ^15.6.0

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Release Notes

netlify/build

v15.6.0

Features

• use a diff instead of a Proxy for configuration mutations (#​3254) (ae7f36e)

Bug Fixes

• deps: update dependency @​netlify/plugins-list to ^2.19.1 (#​3255) (a1db0b0)

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Configuration

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[github-actions]

📊 Benchmark results

Comparing with fd58db1

Package size: 329 MB

⬆️ 0.02% increase vs. fd58db1

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• T (current commit): 329 MB