Fix detection of 'latest' build
Fix detection of 'latest' build

Fixes a race condition where more commits are pushed to master before the
publish step has started running.

e.g. if there's two commits on master since the last tag:
  git describe origin/master --tags
  => v2.0.1-2-gf121d00

Adding --abbrev=0 removes the extra info git adds:
  git describe origin/master --tags --abbrev=0
  => v2.0.1

buildkite/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws