build: asdf-build-system: Make sbcl-* Lisp packages reproducible.

build: asdf-build-system: Make sbcl-* Lisp packages reproducible.

The '*.fasl' files produced by SBCL contain the mtime of the source file (see
commit 72843d7ce32bd615f64a0326cf891658b5724ead of SBCL's code repository).
As asdf-build-system makes a copy of the source files before compiling them,
we must set the mtime of these copies to the same date as the originals for
the build to be reproducible.

* guix/build/asdf-build-system.scm (copy-files-to-output): Keep the
  modification time of the original source files.