fix: set GitHub trigger callback to avoid circular dependency

This should avoid the circular variable dependency error (https://eicweb.phy.anl.gov/EIC/benchmarks/detector_benchmarks/-/pipelines/72824) by defining an empty default value at the origin of the trigger chain.

.gitlab-ci.yml

@@ -4,6 +4,8 @@ variables:
   DETECTOR: epic
   DETECTOR_CONFIG: epic_craterlake
   DETECTOR_REPOSITORYURL: 'https://github.com/eic/epic.git'
+  GITHUB_SHA: ''
+  GITHUB_REPOSITORY: ''
 
 workflow:
   rules: