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.

Ermittle, wo die Sandbox derzeit /bin/sh fest eincodiert und wo die Sandbox-Konfiguration geparst wird. Verfolge, wie der Shell-Einstiegspunkt aufgerufen wird, und überprüfe anschließend, dass der Standardwert /bin/sh bleibt und dass konfigurierte Werte wie /bin/dash oder /bin/bash innerhalb der Build-Sandbox verwendet werden.

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