Fix recompilation of the CI of meson project.

We were assuming that meson project correspond to our projects and so we
were always building them, even if they were already compiled.
(This way, a simple `kiwix-build` is enough to recompile the WIP code of
our project).

However, on the CI, we do not archive the source code/build directory in
the base deps archive. So when we try to compile, the compile step of
meson projects fails because the source are not here.
We have a small workaround for pugixml but as zstd is also meson, it is
time to do something correct.

By default, all projects now try to skip if a build is already present.
Our projects are marked as `force_build` and so, they do not try to skip.

kiwixbuild/dependencies/zim_tools.py

@@ -5,6 +5,7 @@ from .base import (
 
 class ZimTools(Dependency):
     name = "zim-tools"
+    force_build = True
 
     class Source(GitClone):
         git_remote = "https://github.com/openzim/zim-tools.git"