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/actions/run-python-test-set/action.yml

@@ -114,6 +114,7 @@ runs:
         export PLATFORM=${PLATFORM:-github-actions-selfhosted}
         export POSTGRES_DISTRIB_DIR=${POSTGRES_DISTRIB_DIR:-/tmp/neon/pg_install}
         export DEFAULT_PG_VERSION=${PG_VERSION#v}
+        export LD_LIBRARY_PATH=${POSTGRES_DISTRIB_DIR}/v${DEFAULT_PG_VERSION}/lib
 
         if [ "${BUILD_TYPE}" = "remote" ]; then
           export REMOTE_ENV=1