github: reduce parallel tranches to 4 for Postgres-backed itests

CI has shown instability when running Postgres-backed integration tests
with 6 parallel tranches. This change reduces the number of tranches to
4 in an attempt to reduce flakiness and improve test reliability.

Author: ffranr

.github/workflows/main.yaml

@@ -321,7 +321,7 @@ jobs:
           go-version: '${{ env.GO_VERSION }}'
 
       - name: run itest
-        run: make itest-parallel dbbackend=postgres
+        run: make itest-parallel dbbackend=postgres tranches=4
 
       - name: Zip log files on failure
         if: ${{ failure() }}