Fix GHA build workflow (#1120)

This PR fixes the condition to trigger docs-build workflow in build.yaml to:

- simplify the branch assertion
- ensure that it only runs on push events (as opposed to workflow_dispatch events which trigger the nightlies).

@ajschmidt8

Authors:
  - Ajay Thorve (https://github.com/AjayThorve)
  - AJ Schmidt (https://github.com/ajschmidt8)

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

URL: #1120

.github/workflows/build.yaml

@@ -35,7 +35,7 @@ jobs:
       date: ${{ inputs.date }}
       sha: ${{ inputs.sha }}
   docs-build:
-    if: ${{ startsWith(github.ref, 'refs/heads/branch-') }}
+    if: github.ref_type == 'branch' && github.event_name == 'push'
     needs: [conda-python-build]
     secrets: inherit
     uses: rapidsai/shared-action-workflows/.github/workflows/[email protected]