* guix/gexp.scm (computed-file-compiler): Default to (default-guile).
That way, lowering of <computed-file> is not affected by the
'%guile-for-build' parameter.
(file-union): Add #:guile parameter and honor it.
* gnu/packages/ruby.scm (ruby-tzinfo-data): Pass #:guile to 'file-union'.