Builds and returns a list of a desired set of build artifacts
usage: fx get-flash-source [--build] [--allow-empty | --expect-one]
Builds and returns the source file for `fx flash` with an appropriate
prefix. This returns 'flash-manifest:<path>' or 'product-bundle:<path>'
depending on the type of source relevant for the current build
configuration.
--build build the artifacts as well as returning a list of them