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>

.buildkite/scripts/test_old_deps.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 # this script is run by buildkite in a plain `xenial` container; it installs the
 # minimal requirements for tox and hands over to the py35-old tox environment.

.buildkite/scripts/test_synapse_port_db.sh

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Test script for 'synapse_port_db', which creates a virtualenv, installs Synapse along
 # with additional dependencies needed for the test (such as coverage or the PostgreSQL

changelog.d/9689.misc

+Use interpreter from `$PATH` via `/usr/bin/env` instead of absolute paths in various scripts.

contrib/purge_api/purge_history.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 # this script will use the api:
 #    https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_history_api.rst

contrib/purge_api/purge_remote_media.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 DOMAIN=yourserver.tld
 # add this user as admin in your home server:

demo/clean.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

demo/start.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 DIR="$( cd "$( dirname "$0" )" && pwd )"
 

demo/stop.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 DIR="$( cd "$( dirname "$0" )" && pwd )"
 

docker/build_debian.sh

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

docker/run_pg_tests.sh

-#!/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

scripts-dev/check-newsfragment

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # A script which checks that an appropriate news file has been added on this
 # branch.

scripts-dev/config-lint.sh

-#!/bin/bash
+#!/usr/bin/env bash
 # Find linting errors in Synapse's default config file.
 # Exits with 0 if there are no problems, or another code otherwise.
 

scripts-dev/generate_sample_config

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Update/check the docs/sample_config.yaml
 

scripts-dev/lint.sh

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Runs linting scripts over the local Synapse checkout
 # isort - sorts import statements

scripts-dev/make_full_schema.sh

-#!/bin/bash
+#!/usr/bin/env bash
 #
 # This script generates SQL files for creating a brand new Synapse DB with the latest
 # schema, on both SQLite3 and Postgres.

scripts-dev/next_github_number.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 
@@ -6,4 +6,4 @@ set -e
 # next PR number.
 CURRENT_NUMBER=`curl -s "https://api.github.com/repos/matrix-org/synapse/issues?state=all&per_page=1" | jq -r ".[0].number"`
 CURRENT_NUMBER=$((CURRENT_NUMBER+1))
-echo $CURRENT_NUMBER
+echo $CURRENT_NUMBER
\ No newline at end of file

test_postgresql.sh

-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script builds the Docker image to run the PostgreSQL tests, and then runs
 # the tests.