gnu: mcron2: Use Guile 2.2 instead of 2.0.

gnu: mcron2: Use Guile 2.2 instead of 2.0.

Motivation: Using Guile 2.2 instead of Guile 2.0 gives us the scripting
abilities of Guile 2.2 when writing mcron jobs, such as HTTPS support.

* gnu/packages/guile.scm (gnu): Use the srfi-1 module for the alist-delete
function.
(mcron2)[inputs]: Replace guile-2.0 with guile-2.2.
[parallel-build?]: Disable to fix manual page generation.
[phases]: Add a "use-guile-2.2" phase so the build system uses Guile 2.2.
Replace `system*' with `invoke' in the "bootstrap" phase.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>