publish: The public and private keys are now SRFI-39 parameters.

* guix/scripts/publish.scm (%default-options): Add 'public-key-file' and
'private-key-file'.
(lazy-read-file-sexp): Remove.
(%private-key, %public-key): Turn into SRFI-39 parameters.
(signed-string, render-narinfo): Adjust accordingly.
(guix-publish): Honor 'public-key-file' and 'private-key-file' from
OPTS.  Use 'parameterize'.
* guix/pk-crypto.scm (read-file-sexp): New procedure.
* tests/publish.scm: Initialize '%public-key' and '%private-key'.