fix: use portable shebang in shell scripts

Change #!/bin/bash to #!/usr/bin/env bash for better portability
on systems like NixOS where /bin/bash doesn't exist.

Fixes #910

test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 AUTH_FILE="$HOME/.pi/agent/auth.json"