Add an activity for copying sources to the store

Fixes #1184 since it's now visible in the progress bar which path is
taking a long time to copy.

src/libfetchers/input-accessor.cc

@@ -112,6 +112,8 @@ StorePath InputAccessor::fetchToStore(
     } else
         debug("source path '%s' is uncacheable", showPath(path));
 
+    Activity act(*logger, lvlChatty, actUnknown, fmt("copying '%s' to the store", showPath(path)));
+
     auto source = sinkToSource([&](Sink & sink) {
         dumpPath(path, sink, filter ? *filter : defaultPathFilter);
     });