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@279fd02aa7a45c75cba6fb88979b4b5f99bc7f80

gfyoung/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,uschindler/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,robin13/elasticsearch,robin13/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,nknize/elasticsearch,uschindler/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,GlenRSmith/elasticsearch