derivations: Don't memoize 'derivation->bytevector'.

Its hit rate was only 8%.  Removing it reduces heap size of "guix build
libreoffice -nd" from 69MiB to 61MiB and the wall-clock time is unchanged.

* guix/derivations.scm (derivation->bytevector): Change from 'mlambda'
to 'lambda'.

guix/derivations.scm

@@ -622,7 +622,7 @@ (define (write-env-var env-var port)
      (display ")" port))))
 
 (define derivation->bytevector
-  (mlambda (drv)
+  (lambda (drv)
     "Return the external representation of DRV as a UTF-8-encoded string."
     (with-fluids ((%default-port-encoding "UTF-8"))
       (call-with-values open-bytevector-output-port