Fix priority of most important trusted pipelines. (#2795)

- Infra pipelines (create VM/Docker images) run at highest priority 100
since we might need them to fix outages.
- Release builds run at next highest priority 50 - it's the next most
important pipeline which should not be blocked by the backlog of
https://buildkite.com/bazel-trusted/publish-bazel-binaries
- All other pipelines run at default priority 0

Co-authored-by: Xùdōng Yáng <wyverald@gmail.com>