move `build.css` inside /target

[flx-sta]

Describe the bug
build.css is maked as "changed" during every build on windwos due to line-ending changes

To Reproduce
Steps to reproduce the behavior:

1. Have a windows system
2. build the project
3. you will see that the file is marked as changed!

Expected behavior
the build.css file shouldnt be outside /target

Screenshots

Desktop (please complete the following information):

• WIndows

Additional context
https://peregrine-cms.slack.com/archives/CHCV4882U/p1617950955027700