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 using the dotenv variable propagation precedence.

.gitlab-ci.yml

@@ -498,8 +498,6 @@ benchmarks:detector:default:
     - version
     - jug_xl:default
   variables:
-    GITHUB_SHA: "${GITHUB_SHA}"
-    GITHUB_REPOSITORY: "${GITHUB_REPOSITORY}"
     DETECTOR: "epic"
     DETECTOR_CONFIG: epic_craterlake
     DETECTOR_REPOSITORYURL: 'https://github.com/eic/epic.git'