Prioritize fallback over non-interactive mode

[jessarcher]

This PR solves an obscure Windows issue when using mintty without wintty, which causes stream_isatty(STDIN) to incorrectly return false, which triggers non-interactive mode.

This can occur when using "Git Bash" (which uses mintty under the hood) without the php CLI alias. I.e when prefixing a script with php.exe instead of php, or when directly executing a PHP script with a shebang (like the Laravel installer).

I can't find a reliable way to determine whether STDIN is interactive or not in this scenario because mintty simulates a TTY by using a pipe, which is typically not interactive. The most reliable workaround I can think of for the time being is to make the Windows fallback take precedence over non-interactive mode. We could alternatively force-enable interactive mode when using mintty without wintty, but I'm not sure if there is a scenario where this would be genuinely non-interactive.

Fixes laravel/installer#286