distro: Add missing commands in the bootstrap binaries.

* distro/packages/base.scm (%static-inputs): Add an `awk' -> `gawk', and
  `sh' -> `bash' symlinks.  Add `fgrep' and `egrep'.
  (%bootstrap-coreutils&co): Use new tarball that contains that
  symlink.

* distro/packages/bootstrap/x86_64-linux/bash,
  distro/packages/bootstrap/x86_64-linux/mkdir,
  distro/packages/bootstrap/x86_64-linux/tar,
  distro/packages/bootstrap/x86_64-linux/xz: Use binaries from that
  tarball (the previous binaries hadn't gone through
  `remove-store-references', which is fixed now.)