bug#44924: guix build u-boot-pinebook --with-git-url failed due to "source" in C_INCLUDE_PATH

[宋文武]

> LGTM!
>
> You can push it right away to ‘core-updates’, but since it triggers a
> full rebuild, now’s a good time to push another change that triggers a
> full rebuild.  :-)
>

Pushed bravely, thank you!