* guix/build-system/trivial.scm (lower): Add #:allowed-references and
keep it in the 'arguments' field.
(trivial-build): Add #:allowed-references. Add
'canonicalize-reference'. Pass #:allowed-references to
'build-expression->derivation'.
(trivial-cross-build): Likewise.
* tests/packages.scm ("trivial with #:allowed-references"): New test.