In SKIP_QUIET mode, pass --warn to Gradle, skip info logging

[dfabulich]

I missed a crucial spot in #655, because on a rebuild, Gradle output is often suppressed. Now I've tested it with clean build and it's much quieter.

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Cursor did this one; I manually cleaned it up a bit. I tested the build with and without SKIP_QUIET. I've run swift test here in skip and also copied the changes to skipstone and ran swift test there, too.