feat: Support Bash on Windows using 'pixi shell'

[mwiebe]

This depends on the PR conda/rattler#1392 for the bash shell hook to work correctly on Windows.

Create a function start_winbash for running bash on Windows that launches bash using the command:

$ bash --init-file <temp-shell-hook> -i

Here's an example output where I've built pixi against the rattler PR rebased to an older rattler commit that is compatible with the pixi main branch:

mw@Home MINGW64 /c/Dev/pixi (win-git-bash)
$ echo $PATH
/c/Users/mw/bin:/mingw64/bin:/usr/local/bin:/usr/bin:/bin:/mingw64/bin:/usr/bin:/c/Users/mw/bin:...

mw@Home MINGW64 /c/Dev/pixi (win-git-bash)
$ ./target/debug/pixi.exe shell
(pixi)
mw@Home MINGW64 /c/Dev/pixi (win-git-bash)
$ echo $PATH
/c/Dev/pixi/.pixi/envs/default:/c/Dev/pixi/.pixi/envs/default/Library/mingw-w64/bin:
/c/Dev/pixi/.pixi/envs/default/Library/usr/bin:/c/Dev/pixi/.pixi/envs/default/Library/bin:
/c/Dev/pixi/.pixi/envs/default/Scripts:/c/Dev/pixi/.pixi/envs/default/bin:/c/Users/mw/bin:...
(pixi)
mw@Home MINGW64 /c/Dev/pixi (win-git-bash)
$ echo $CONDA_PREFIX
C:\Dev\pixi\.pixi\envs\default