* 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.