build: Don't hardcode the path of bash

Bash is not always installed in /bin and we should not hardcode the path
of it in source code which is expected to be built on many operating
systems and distributions.

Since most scripts using #!/bin/bash here doesn't have any bashism,
they can be converted to #!/bin/sh instead of using /usr/bin/env trick.

clutter/tests/interactive/meson/gen-test-unit-names.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 outputfile=$1
 shift

cogl/tests/conform/meson/find-conform-unit-tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 inputfile="$1"
 outputfile="$2"

cogl/tests/run-tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 if test -z "$G_DEBUG"; then
     G_DEBUG=fatal-warnings

cogl/tests/unit/meson/find-unit-tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 inputfile="$1"
 outputfile="$2"