The build rules were highly inspired by other projects on GNOME's
Gitlab. All of them used to build with ccache. It worked fine for the
main build, but completely broke GObject Introspection build on both
babl and GEGL. And the worse thing is that meson was absolutely not
displaying the error, just saying it failed (even in verbose mode). A
lot of time wasted trying to debug.

Therefore let's get rid of ccache, but only for babl and GEGL. Keep it
for GIMP itself as it works fine there.

Other minor changes:

* Build from the build dir, rather than source. The other way around
  works too, but I actually find commands simpler this way.
* Adding artifacts.