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>

docs/en/hardware/board_packaging.md

@@ -159,7 +159,7 @@ Runs after the package is installed. Common tasks:
 - Board-specific setup (e.g., DSP signature generation)
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # Create px4-* symlinks
@@ -185,7 +185,7 @@ fi
 Runs before the package is removed:
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # Stop the service