Push build.snapshot to BWC builds too
Push build.snapshot to BWC builds too

We need to push this flag down to the command line for BWC builds or the
artifacts in release tests will have the wrong version (being considered
snapshots instead of non-snapshots).

Original commit: elastic/x-pack-elasticsearch@6d875da941011300df3b22c14de7084b8207fe95

vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra