Build concurrency for nightly and merge triggers (#688)

This PR separates the concurrency groups for nightly and PR merge triggers, ensuring that nightlies aren't interrupted by PR merges canceling jobs.

See rapidsai/ops#2932.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: #688

.github/workflows/build.yaml

@@ -22,7 +22,7 @@ on:
         default: nightly
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
   cancel-in-progress: true
 
 jobs: