* gnu/ci.scm (channel-build-system)[build, lower]: Honor #:source.
(channel-source->package): New procedure.
(system-test-jobs): Remove 'instance' and call to
'checkout->channel-instance'. Use 'channel-source->package'.
* build-aux/run-system-tests.scm (tests-for-channel-instance): Rename to...
(tests-for-current-guix): ... this. Change 'instance' to 'source'.
(run-system-tests): Use 'local-file' instead of 'interned-file' for
SOURCE.