This adds logic to download and use the sky_snapshot binary from
Google cloud storage when running the 'sky_tools build' command.
The downloaded binary is put into lib/cache/... The binary is
chosen to match the REVISION in the sky_engine package in the
packages directory of whichever package the user wishes to
build a flx from.
Known issues:
*) Assumes linux-x64 host
*) Assumes download will always produce valid executable
*) No clearing of stale cache entries
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