Currently, if (@ (guix build utils) wrap-program) is called multiple
times for the same file, the original file ends up being overwritten.
This happened to me when trying to wrap a python program, which had
already once been wrapped by python-build-system.  The
python-build-system wrapper sets PYTHON_PATH, and I needed to wrap the
program again in order to set PATH.

Comments are very welcome on this patch to core-updates, as I hacked it
together rather quickly.

A description of what ends up happening, e.g.:

1) Initially::

  $ ls
  foo

2) Then after first call to wrap-program::

  $ ls
  foo -> ./.foo-wrap-01
  .foo-real
  .foo-wrap-01

3) And then after another call to wrap-program::

  $ ls
  foo -> ./.foo-wrap-02
  .foo-real
  .foo-wrap-01
  .foo-wrap-02