pipeline: checkpoint assemblers just like stages

Change the assembler-commit to be conditional on checkpoints, just like
we already do for stages. This means, assembler output is not
automatically committed, but only if you requested so via a checkpoint.

With this in place we can start sharing caches in osbuild-composer. The
only thing in the cache will be sources as well as checkpointed stages.
We can start checkpointing known pipelines and thus make use of the
cache. Furthermore, we can cache sources, as long as we do not fetch an
unbound set of RPMs. However, our RPM set is currently static, so this
should not be an issue. Nevertheless, it is up to Composer to decide
when to enable the cache.

osbuild/main_cli.py

@@ -30,9 +30,17 @@ def mark_checkpoints(pipeline, checkpoints):
             stage.checkpoint = True
             points.remove(c)
 
+    def mark_assembler(assembler):
+        c = assembler.id
+        if c in points:
+            assembler.checkpoint = True
+            points.remove(c)
+
     def mark_pipeline(pl):
         for stage in pl.stages:
             mark_stage(stage)
+        if pl.assembler:
+            mark_assembler(pl.assembler)
         if pl.build:
             mark_pipeline(pl.build)