fix(scripts): replace hardcoded /bin/bash shebangs

Several helper scripts assumes bash is available at /bin/bash. That breaks on systems
such as NixOS, where bash is resolved from PATH instead of a fixed /bin location and
causes failures like `bad interpreter` during `make format`, e.g., on my host machine:

```sh
$ make format

/PX4-Autopilot/Tools/astyle/check_code_style.sh: /PX4-Autopilot/Tools/astyle/fix_code_style.sh: /bin/bash: bad interpreter: No such file or directory
```

This change switches these entrypoints to `#!/usr/bin/env bash` so they locate bash properly.

No functional changes intended.

Signed-off-by: Onur Özkan <work@onurozkan.dev>

Tools/simulation/gazebo-classic/setup_gazebo.bash

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Setup environment to make PX4 visible to Gazebo.
 #

Tools/simulation/gazebo-classic/sitl_multiple_run.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # run multiple instances of the 'px4' binary, with the gazebo SITL simulation
 # It assumes px4 is already built, with 'make px4_sitl_default sitl_gazebo-classic'
 

Tools/simulation/sitl_multiple_run.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Run multiple instances of the 'px4' binary, without starting an external simulator.
 # It assumes px4 is already built with the specified build target.
 #