Trivial build system

[Andreas Enge <andreas@enge.fr>]

Hello,

in my work on texlive, I am trying to build a package by simply applying
the 'unpack and 'patch-source-shebangs phases of gnu-build-system. A working
approach is to delete all other phases from gnu-build-system, but that would
break the package whenever gnu-build-system is augmented. So I tried to
start from trivial-build-system. But the following code fails:

   (arguments
    `(#:modules ((guix build gnu-build-system)
                 (guix build utils))
      #:builder (begin
                  (use-modules (guix build utils)
                               (guix build gnu-build-system))
                  (let ((out %output)
                        (source (assoc-ref %build-inputs "source"))
                        (unpack (assoc-ref %standard-phases 'unpack)))
                    (format #t "XXX ~a XXX\n" source)
                    (apply unpack (list #:source source))
                    (mkdir-p out)
                    (zero? (system* "mv" "tlpkg" out))
                    ))))

The error message is
In execvp of tar: No such file or directory
In execvp of mv: No such file or directory
XXX /gnu/store/xrm2iibfw8x38b9jjkwbidgw96b6aa6i-texlive-20140525-extra.tar.xz XXX
builder for `/gnu/store/q5w5qp1nh6i1275xkggidpy65fsyifvi-texlive-texmf-2014.drv' failed with exit code 1

I printed the value of SOURCE which contains indeed the source to unpack.
What is my mistake here?

Andreas