WorkingDirectoryBehavior::Preserve shouldn't error on copy of the exists file

[nichmor]

Currently setting WorkingDirectoryBehavior::Preserver, will error out if we try to build the same pkg again, with the error:

× FileSystem error: 'Path "/Users/graf/oss/pixi-build-testsuite/pytest-temp/px0/.pixi/build/work/array-api-extra-WjfygdhaEew/work/RELEASING.md" already exists!'

I think we can maybe ignore the errors in copying when specifically WorkingDirectoryBehavior::Preserve is set.

[baszalmstra]

You can also have the situation where two source definitions would otherwise overwrite the content of the other. That might not be desired behavior.