Until now, 'current-guix-package' was parameterized in the wrong
context. Thus, 'current-guix' would end up building a variant of the
'guix' package instead of the package returned by
'channel-source->package', which is much less expensive to build.
* etc/system-tests.scm (mparameterize): New macro.
(tests-for-current-guix): Change the 'value' field of each <system-test>
record to parameterize 'current-guix-package' for good.