gnu: commencement: Ensure 'gnu-make-final' refers to the native 'pkg-config'.

gnu: commencement: Ensure 'gnu-make-final' refers to the native 'pkg-config'.

Fixes <https://bugs.gnu.org/38093>.

When running:

  guix build --target=arm-linux-gnueabihf -e '(@ (gnu packages base) coreutils)'

the '%current-target-system' parameter is set by the time the top-level
of (gnu packages commencement) is evaluated.  Consequently, we need to
ensure that the 'pkg-config' macro evaluates in a context where
'%current-target-system' is unset.

* gnu/packages/commencement.scm (gnu-make-final): Refer to '%pkg-config'
instead of 'pkg-config'.
* tests/guix-build.sh: Add test.