Merge pull request #1842 from FluxML/bc/codecov

Add codecov on CI

Author: mcabbott

.buildkite/pipeline.yml

@@ -14,6 +14,8 @@ steps:
       - JuliaCI/julia#v1:
           version: "1"
       - JuliaCI/julia-test#v1: ~
+      - JuliaCI/julia-coverage#v1:
+          codecov: true
     agents:
       queue: "juliagpu"
       cuda: "*"