Always output bash-style environment variable file even if running on windows

[robertbrignull]

We potentially output files with environment variables in formats for a few different shells, but we base that decision solely on which OS we're running on. This leads to problems if people use bash on windows because in that case we currently only output files for cmd and powershell. We should consider always outputting the bash-style file. We're probably fine to still only do the cmd and powershell ones on windows as I can't see anybody using either of those on another OS.