monitor: fix duration calculation

When fully cached manifest is used, the duration used to print an
incorrect value of 55 years:

    python3 -m osbuild --libdir . ./test/data/manifests/fedora-boot.json
    ⏱  Duration: 1753191578s

This patch fixes the duration calculation to use the correct timestamp
from the manifest by using monotonic timer instead. Additionally, it
prints nothing when there was no module executed. Finally, it improves
the formatting of the duration output.

test/mod/test_monitor.py

@@ -57,6 +57,31 @@ class TapeMonitor(osbuild.monitor.BaseMonitor):
 
 
 class TestMonitor(unittest.TestCase):
+    @unittest.skipUnless(test.TestBase.can_bind_mount(), "root-only")
+    def test_log_monitor_no_duration(self):
+        index = osbuild.meta.Index(os.curdir)
+
+        runner = Runner(index.detect_host_runner())
+        pipeline = osbuild.Pipeline("pipeline", runner=runner)
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            storedir = os.path.join(tmpdir, "store")
+
+            logfile = os.path.join(tmpdir, "log.txt")
+
+            with open(logfile, "w", encoding="utf8") as log, ObjectStore(storedir) as store:
+                monitor = LogMonitor(log.fileno())
+                res = pipeline.run(store,
+                                   monitor,
+                                   libdir=os.path.abspath(os.curdir))
+                monitor.result(res)
+
+                with open(logfile, encoding="utf8") as f:
+                    log = f.read()
+
+            assert res
+            self.assertNotIn("Duration", log)
+
     @unittest.skipUnless(test.TestBase.can_bind_mount(), "root-only")
     def test_log_monitor_vfuncs(self):
         # Checks the basic functioning of the LogMonitor
@@ -86,6 +111,7 @@ class TestMonitor(unittest.TestCase):
             assert res
             self.assertIn(pipeline.stages[0].id, log)
             self.assertIn("isthisthereallife", log)
+            self.assertIn("Duration", log)
 
     @unittest.skipUnless(test.TestBase.can_bind_mount(), "root-only")
     def test_monitor_integration(self):