Feature/additional activation scripts

[ruben-arts]

Add the ability to add activation scripts to the pixi.toml this is improtant for ROS project for example.

[activation]
scripts = [".pixi/install/setup.sh"]

[target.win-64.activation]
scripts = [".pixi/install/setup.ps1"]

NOTE: if a script is not found it will warn but not error, and like in this example the scripts variable will be overwritten by the target version if it is available for the current platform.

[wolfv]

I like it better! :)