googlesource's +archive tarballs are generated per request: their bytes
change on every download, so the recipe skipped their checksums, and the
endpoint refuses CI runners at times (503s on aom failed two #785
builds). Clone both at their pinned commits instead, which git verifies.
_prepare_vc writes downloads/<port>-<ref>.tar.gz with the same git
archive command vcpkg_from_git uses, so vcpkg finds them cached. The
archives hold exactly the files of the old tarballs (aom 1347, libyuv
183).
Emir Beganovic
c050643155