CI(build-tools): Remove libpq from build image (#8206)

## Problem
We use `build-tools` image as a base image to build other images, and it
has a pretty old `libpq-dev` installed (v13; it wasn't that old until I
removed system Postgres 14 from `build-tools` image in
https://github.com/neondatabase/neon/pull/6540)

## Summary of changes
- Remove `libpq-dev` from `build-tools` image
- Set `LD_LIBRARY_PATH` for tests (for different Postgres binaries that
we use, like psql and pgbench)
- Set `PQ_LIB_DIR` to build Storage Controller
- Set `LD_LIBRARY_PATH`/`DYLD_LIBRARY_PATH` in the Storage Controller
where it calls Postgres binaries

.github/workflows/benchmarking.yml

@@ -379,6 +379,10 @@ jobs:
 
     - name: Add Postgres binaries to PATH
       run: |
+        LD_LIBRARY_PATH="${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/lib"
+        export LD_LIBRARY_PATH
+        echo "LD_LIBRARY_PATH=${LD_LIBRARY_PATH}" >> $GITHUB_ENV
+
         ${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/bin/pgbench --version
         echo "${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/bin" >> $GITHUB_PATH