This works around an occasional issue where substitution stops abruptly
due to “Error in the push function” from GnuTLS, as reported at
<https://issues.guix.gnu.org/71238> by Richard Sent.
* guix/scripts/substitute.scm (call-with-cached-connection): Add
‘error/push-error’ and ‘error/pull-error’ to the list of gnutls-error
values for which the connection is reopened.
Change-Id: Icf079dd10b16739f62fee15bc3d90bab77110576