build: export GIT_CEILING_DIRECTORIES for package builds

A package may run git as part of its build process, and if the package
source code is not from a git checkout, then git may traverse up the
directory tree to find buildroot's repository directory (.git).

For instance, Poetry Core, a Python build backend, will read the
contents of .gitignore for paths to exclude when creating a Python
package. If it finds buildroot's .gitignore file, then Poetry Core will
exclude all of the package's files[1].

This exports GIT_CEILING_DIRECTORIES for both package and host builds so
that git will not traverse beyond $(BUILD_DIR)/$(BUILD_DIR_HOST).

[1]: https://github.com/python-poetry/poetry/issues/5547

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

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