Use interpreter from $PATH instead of absolute paths in various scripts using /usr/bin/env (#9689)

On NixOS, `bash` isn't under `/bin/bash` but rather in some directory in `$PATH`. Locally, I've been patching those scripts to make them work.

`/usr/bin/env` seems to be the only [portable way](https://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my) to use binaries from the PATH as interpreters.

Signed-off-by: Quentin Gliech <quentingliech@gmail.com>

docker/build_debian.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # The script to build the Debian package, as ran inside the Docker image.
 

docker/run_pg_tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script runs the PostgreSQL tests inside a Docker container. It expects
 # the relevant source files to be mounted into /src (done automatically by the