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

[宋文武]

Running:
  guix build u-boot-pinebook \
    --with-git-url=u-boot-pinebook=https://github.com/u-boot/u-boot

Would fail with "fatal error: asm/string.h: No such file or directory".

And it's caused by the first entry in C_INCLUDE_PATH, which is
"/gnu/store/xxx-u-boot-xxx/include".

I think we should filter out "source" in `set-paths` of the
`gnu-build-system`.