build-system/{gnu,trivial}: Add a `#:guile' keyword parameter.

* guix/build-system/gnu.scm (package-with-explicit-inputs): New `guile'
  keyword parameter.  Add it to P's arguments, and pass it in recursive
  calls.
  (gnu-build): New `guile' keyword parameter; new `guile-for-build'
  variable.  Pass it as the `#:guile-for-build' parameter of
  `build-expression->derivation'.

* guix/build-system/trivial.scm (trivial-build): Likewise.