gnu: bootstrap: Download the bootstrap bash, mkdir, tar, and xz binaries.

gnu: bootstrap: Download the bootstrap bash, mkdir, tar, and xz binaries.

* gnu/packages/bootstrap.scm (%bootstrap-executables): New variable.
(bootstrap-executable-url, bootstrap-executable): New procedure.
(raw-build)[->store]: Use 'run-with-store' and 'origin->derivation'.
Add calls to 'derivation->output-path', and remove the list of
references passed to 'add-text-to-store' for BUILDER.  Augment the list
of #:inputs passed to 'derivation'.
(package-from-tarball): Use 'bootstrap-executable' instead of
'search-bootstrap-binary'.
(%bootstrap-glibc, %bootstrap-gcc, %bootstrap-mescc-tools)
(%bootstrap-mes): Likewise.
* guix/scripts/environment.scm (environment-bash): Use
'bootstrap-executable' instead of 'search-bootstrap-binary'.
(guix-environment): Adjust CONTAINER? case accordingly.
* po/guix/POTFILES.in: Add gnu/packages/bootstrap.scm.