Make sandbox shell configurable

Currently sandbox shell is hardcoded to be /bin/sh. This is problematic since it can wildly vary how it works. If system has bash as primary shell, which is common, then /bin/sh will be a very much extended version of POSIX shell so many scripts written with it will not be compatible with eg dash. On the other hand, it is also not full bash. Many projects like freedesktop-sdk expect to be able to write bash syntax in command so it would really be ideal that sandbox grows a variable `shell` with default value set to `/bin/sh` for backwards compatibility. When set to eg `/bin/dash` or `/bin/bash`, then that entrypoint will be used inside build sandbox.

Individua dove la sandbox codifica attualmente /bin/sh in modo fisso e dove viene analizzata la configurazione della sandbox. Traccia come viene invocato l’entrypoint della shell, quindi verifica che il valore predefinito rimanga /bin/sh e che i valori configurati, come /bin/dash o /bin/bash, vengano utilizzati all’interno della sandbox di build.

Scritto dal modello di indicizzazione a partire dal testo della issue.