serialization: Adjust the permissive UTF-8 decoder to Guile 2.0.12ish.

* guix/serialization.scm (read-maybe-utf8-string): Use
  'set-port-encoding!' and 'set-port-conversion-strategy!' instead of
  setting '%default-port-encoding' and
  '%default-port-conversion-strategy'.  This accounts for Guile commit
  d574d96, which changes bytevector input ports to use ISO-8859-1.