pixi build shouldn't rely on gitignore

[tdejager]

If you have e.g. a pixi-build-python backend in a pyproject.toml and run pixi install you will get a “filename too long” error if condition is not fulfilled:

• .gitignore file with “.pixi” entry

This happens since rattler-build gets into a recursive loop grabbing everything. We should add a way to inform rattler-build which directories it should ignore