ci: only publish benchmarks nightly

Allow for workflow_dispatch jobs to run, but don't publish their test
results; this is useful for testing workflows themselves.

Author: ethomson

.github/workflows/benchmark.yml

@@ -97,7 +97,7 @@ jobs:
   publish:
     name: Publish results
     needs: [ build ]
-    if: always() && github.repository == 'libgit2/libgit2'
+    if: always() && github.repository == 'libgit2/libgit2' && github.event_name == 'schedule'
     runs-on: ubuntu-latest
     steps:
     - name: Check out benchmark repository