Do not assume bash is in /bin

Currently `custom-script-shim` use `/bin/bash` as interpreter but on some distro (notably NixOS) this is not where bash is available.

Could `#!/usr/bin/env bash` be used instead?

`custom-script-extension` may also suffer from a similar problem as it fails with
```
-bash: /var/lib/waagent/Microsoft.Azure.Extensions.CustomScript-2.0.6/bin/custom-script-extension: No such file or directory
```
But since it's a blob I am not sure how to verify that.