ci: Use a valid 'current-guix'.

ci: Use a valid 'current-guix'.

This fixes a regression introduced in
b5f8c2c88543158e8aca76aa98f9009f6b9e743a whereby 'current-guix' (needed
by some of the system tests) would fail to build.
Reported by Ricardo Wurmus <rekado@elephly.net>.

It also speeds up compilation of 'current-guix' since the channel
instance is already compiled or can be built quickly compared to the
default 'current-guix'.

* gnu/packages/package-management.scm (current-guix-package): New
variable.
(current-guix): Honor it.
* gnu/ci.scm (channel-build-system): New variable.
(channel-instances->derivation): New procedure.
(system-test-jobs): Add #:source and #:commit parameters.
Define 'instance' and parameterize CURRENT-GUIX-PACKAGE.
(hydra-jobs)[checkout, commit, source]: New variables.
Pass #:source and #:commit to 'system-test-jobs'.