Use a simpler way to set extra options for benchmark test.

Commit 43a4f7173e fixed the case that there are extra options in the
connection string, but broke it in the case when there are not. Fix
that. But on second thoughts, it's more straightforward set the
options with ALTER DATABASE, so change the workflow yaml file to do
that instead.

.github/workflows/benchmarking.yml

@@ -183,12 +183,9 @@ jobs:
           neon-captest-reuse)
             CONNSTR=${{ secrets.BENCHMARK_CAPTEST_CONNSTR }}
             ;;
-          neon-captest-new)
+          neon-captest-new | neon-captest-prefetch)
             CONNSTR=${{ steps.create-neon-project.outputs.dsn }}
             ;;
-         neon-captest-prefetch)
-            CONNSTR=${{ steps.create-neon-project.outputs.dsn }}?options=-cenable_seqscan_prefetch%3Don%20-cseqscan_prefetch_buffers%3D10
-            ;;
           rds-aurora)
             CONNSTR=${{ secrets.BENCHMARK_RDS_CONNSTR }}
             ;;
@@ -204,6 +201,14 @@ jobs:
       env:
         PLATFORM: ${{ matrix.platform }}
 
+    - name: Set database options
+      if: matrix.platform == 'neon-captest-prefetch'
+      run: |
+        psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE main SET enable_seqscan_prefetch=on"
+        psql ${BENCHMARK_CONNSTR} -c "ALTER DATABASE main SET seqscan_prefetch_buffers=10"
+      env:
+        BENCHMARK_CONNSTR: ${{ steps.set-up-connstr.outputs.connstr }}
+
     - name: Benchmark init
       uses: ./.github/actions/run-python-test-set
       with: