Fixes <https://issues.guix.gnu.org/67575>.
* guix/scripts/substitute.scm (process-substitution/fallback): Use
‘report-error’ instead of ‘leave’. Write status line to PORT.
* tests/substitute.scm ("substitute, narinfo is available but nar is
missing"): Adjust accordingly.
Change-Id: Ic7297dbd563c007111ec2167c8d52505a07d4822