Add and propagate quiet arg to build_site()

[jayhesselberth]

Closes #2330

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[jayhesselberth]

There's also expect_snapshot(variant = ...) but that seems like even more work for this one test.

A simpler test would be to check for "pandoc" or "chunk" in the message output, which I don't think we'd ever see unless the rmarkdown build output was included (by quiet = FALSE).

[ethanbass]

For what it's worth, from my point of view I think that testing for some key phrases like "pandoc" and/or "chunk" seems like a reasonable approach for testing whether the quiet is being passed through correctly to the build functions.