build-system/{gnu,trivial-build}: Fix handling of #:guile argument.

* guix/build-system/gnu.scm (gnu-build)[guile-for-build]: Check whether
  GUILE matches string? before checking whether it matches
  derivation-path?.
* guix/build-system/trivial.scm (trivial-build)[guile-for-build]:
  Likewise.