Fix hardcoded "Linux" in web search fallback

Jaimukund Bhanrequested to merge
jaimukund/discover:work/jaimukund/513366 into master

Previously, the "Search the Web" button hardcoded "Linux" into the DuckDuckGo search query, which was inaccurate for users on FreeBSD. This change uses Qt.platform.os to dynamically insert the operating system name into the search query.

BUG: 513366

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