[now dev] Pass in the builder-specific `workPath` to `shouldServe()` (#2214)
[now dev] Pass in the builder-specific `workPath` to `shouldServe()` (#2214)

The dev server `cwd` is not correct working path of a build match, so
pass in the correct one. This property will be used by
`@now/static-build`'s dev mode.
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