dev/ci: sanitize + from branch names for tag

Created by: bobheadxi

Fix for https://sourcegraph.slack.com/archives/C01N83PS4TU/p1655150576099319?thread_ts=1655145763.437259&cid=C01N83PS4TU

Test plan

sg ci build docker-images-patch-notest jaeger-agent

https://buildkite.com/sourcegraph/sourcegraph/builds/154297