utils: 'wrap-program' produces only one wrapper file.

* guix/build/utils.scm (wrap-program)[wrapper-file-name]
[next-wrapper-number, wrapper-target]: Remove.
[wrapped-file, already-wrapped?]: New variables.
[last-line]: New procedure.
Use it to append to PROG when a wrapper already exists.
* tests/build-utils.scm ("wrap-program, one input, multiple calls"):
Adjust the list of files to delete.