Temporarily use vanilla pgbench and psql (client) for running pgvector benchmark (#8422)

## Problem

https://github.com/neondatabase/neon/issues/8275 is not yet fixed

Periodic benchmarking fails with SIGABRT in pgvector step, see
https://github.com/neondatabase/neon/actions/runs/9967453263/job/27541159738#step:7:393

## Summary of changes

Instead of using pgbench and psql from Neon artifacts, download vanilla
postgres binaries into the container and use those to run the client
side of the test.

.github/workflows/benchmarking.yml

@@ -457,17 +457,21 @@ jobs:
     runs-on: [ self-hosted, us-east-2, x64 ]
     container:
       image: 369495373322.dkr.ecr.eu-central-1.amazonaws.com/build-tools:pinned
-      options: --init
+      options: --init --user root
 
     steps:
     - uses: actions/checkout@v4
 
-    - name: Download Neon artifact
-      uses: ./.github/actions/download
-      with:
-        name: neon-${{ runner.os }}-${{ runner.arch }}-release-artifact
-        path: /tmp/neon/
-        prefix: latest
+    # until https://github.com/neondatabase/neon/issues/8275 is fixed we temporarily install postgresql-16
+    # instead of using Neon artifacts containing pgbench
+    - name: Install postgresql-16 where pytest expects it
+      run: |
+        apt-get update && apt-get install -y postgresql-common
+        /usr/share/postgresql-common/pgdg/apt.postgresql.org.sh -y
+        apt-get -y install postgresql-16
+        mkdir -p /tmp/neon/pg_install/v16/bin
+        ln -s /usr/bin/pgbench /tmp/neon/pg_install/v16/bin/pgbench
+        ln -s /usr/bin/psql /tmp/neon/pg_install/v16/bin/psql
 
     - name: Set up Connection String
       id: set-up-connstr