pack: Do not cross-compile the image building tools.

Until now, something like:

  guix pack -f docker --target=arm-linux-gnueabihf

would attempt to cross-compile tar.

* guix/scripts/pack.scm (self-contained-tarball): Pass #:target to
'gexp->derivation'.
(squashfs-image): Use #+ instead of #$ for ARCHIVER.  Use 'file-append'
instead of 'string-append'.  Pass #:target to 'gexp->derivation'.
(docker-image): Likewise.