windows doesn't respect the shell

**Describe the bug**
Unlike on macos where nushell works fine, windows is [hardcoded](https://github.com/jesseduffield/lazygit/blob/master/pkg/commands/oscommands/os_windows.go) to use cmd

**To Reproduce**
I discovered it here: https://github.com/jesseduffield/lazygit/issues/4047#issuecomment-2954067602
It wasn't obvious at first but after debugging it I realised it was using `cmd /c` despite `$env.SHELL` being set.
Then I tracked it back to https://github.com/jesseduffield/lazygit/pull/4385 and discovered the hardcoded case for windows.

**Expected behavior**
To work as other os, I specifically use nushell because of how powerful it is at being cross platform.

Ultimately for me this whole shell thing shouldn't be hardcoded or only fallbacks and expose both as options

**Version info:**
No sure why but building from master I only get:

```
commit=, build date=, build source=unknown, version=unversioned, os=windows, arch=amd64, git version=2.49.0.windows.1
```