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>

scripts-dev/check-newsfragment

@@ -1,4 +1,4 @@
-#!/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

@@ -1,4 +1,4 @@
-#!/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

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Update/check the docs/sample_config.yaml
 

scripts-dev/lint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Runs linting scripts over the local Synapse checkout
 # isort - sorts import statements

scripts-dev/make_full_schema.sh

@@ -1,4 +1,4 @@
-#!/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

@@ -1,4 +1,4 @@
-#!/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