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.

Locate where the sandbox currently hardcodes /bin/sh and where sandbox configuration is parsed. Trace how the shell entrypoint is invoked, then verify that the default remains /bin/sh and that configured values such as /bin/dash or /bin/bash are used inside the build sandbox.

Written by the indexing model from the issue text.