The current rest backcompat tests, which run against a mixed cluster of
5.x and 6.0 nodes, depend on snapshot builds of 5.x. However, this has
the potential for inconsistency that results in CI failures, and happens
quite often, whenever some backcompat logic is added to 5.x, but the bwc
test on master fails because the 5.x code has not yet been published as
a snapshot.
This change creates a git clone of the 5.x branch,
builds the zip distribution, and ties that into gradle substitutions for
the 5.x version.
8c53555b28