Add checkout step to reusable-fabric-etl workflow (#25)

Phase 1 extracted inline Python from this workflow into scripts/run_fabric_etl.py
but the corresponding actions/checkout step was never added. The other two
refactored reusables (reusable-deploy-supported.yml, reusable-deploy-bulk.yml)
both have it; only this one was missed.

Without checkout, the runner has no scripts/ directory and the python invocation
fails with 'No such file or directory'. The Phase 1 refactor's test-environment
ETL run looked successful because workflow_run-triggered workflows execute the
workflow file from the default branch — at that moment main still held the
pre-refactor inline-Python version. The bug surfaced as soon as PR #24 promoted
the refactored workflow to main, breaking the next prod ETL run.

Author: michaeldeongreen

.github/workflows/reusable-fabric-etl.yml

@@ -51,6 +51,9 @@ jobs:
     timeout-minutes: 60
     environment: ${{ inputs.environment }}
     steps:
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
       - name: Set up Python
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6
         with: