feat: add isolated build tools

[nichmor]

• This PR aims to add support of isolated build tools. This means that user don't need anymore to have some of the build backends installed in the system, pixi will take care of it.
• we also introduce --with-system option for pixi build. This mean that pixi instead of installing these tools, will use the one present in system. It was not possible to use these features previously.
• it also introduce channels in build table. This means that user can specify some private / or different channels to get their build tools. This is also handy for us because we host currently our build tools on pixi-build-backends channel.

It cache the env based on the command, spec and channels.

This is also a small demo of end-2-end result

Screen.Recording.2024-11-13.at.18.57.38.mov

[tdejager]

Would be great to add the following follow-up PR's:

• Re-use environments if they are already installed.
• Report progress when installing build tools.