system: Add (guix build install) module.

* guix/build/vm.scm (install-grub, evaluate-populate-directive,
  reset-timestamps, register-closure): Move to...
* guix/build/install.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add (guix
  build install) to #:modules.