Use /bin/sh as command line interpreter

[manicmaniac]

ShellExecutor considers $SHELL environment variable to decide which shell should be used.
Actually POSIX permits to set $SHELL to any kind of command line interpreter even it doesn't conform to the specification for shell command interpreter.

https://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html
SHELL
This variable shall represent a pathname of the user's preferred command language interpreter. If this interpreter does not conform to the Shell Command Language in the Shell and Utilities volume of IEEE Std 1003.1-2001, Chapter 2, Shell Command Language, utilities may behave differently from those described in IEEE Std 1003.1-2001.

Potentially users even can do sudo chsh -s /usr/bin/python and in that case, danger-swift doesn't work properly.

Therefore I changed ShellExecutor not to depend on the environment variable but just use POSIX standard /bin/sh.
/bin/sh is guaranteed to be there and it must be a bsh-compatible shell script language interpreter.

https://pubs.opengroup.org/onlinepubs/7908799/xcu/sh.html

[f-meloni]

Nice! Thank you! :)

[peril-staging]

Thanks for the PR @manicmaniac.

This PR has been shipped in v2.0.2 - CHANGELOG.