fix(ci): serialize BigQuery test runs to prevent table race condition

Two Main workflow runs fire simultaneously on release (tag + branch push
for the same commit). Both setup-unit.sh/cleanup-unit.sh touch shared BQ
tables, causing 404 errors when one run's cleanup races the other's tests.

Author: fdelbrayelle

.github/workflows/main.yml

@@ -33,6 +33,10 @@ on:
         type: string
         default: ''
 
+concurrency:
+  group: bigquery-unit-tests
+  cancel-in-progress: false
+
 permissions:
   contents: write
   checks: write