derivations: Export 'derivation-builder'.

* guix/derivations.scm: Export 'derivation-builder'.

guix/derivations.scm

@@ -37,6 +37,7 @@ (define-module (guix derivations)
             derivation-inputs
             derivation-sources
             derivation-system
+            derivation-builder
             derivation-builder-arguments
             derivation-builder-environment-vars
             derivation-file-name