libstore: reduce loglevel of waiting for a machine to build

This comes quite often when the available job slots on all remote
builders are exhausted and this is pretty spammy.

This isn't really an issue, but expected behavior.

A better way to display this is a nom-like approach where all scheduled
builds are shown in a tree and pending builds are being marked as such
IMHO.

Change-Id: I6bc14e6054f84e3eb0768127b490e263d8cdcf89

src/libstore/build/derivation-goal.cc

@@ -760,7 +760,7 @@ void DerivationGoal::tryToBuild()
                 /* Not now; wait until at least one child finishes or
                    the wake-up timeout expires. */
                 if (!actLock)
-                    actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
+                    actLock = std::make_unique<Activity>(*logger, lvlTalkative, actBuildWaiting,
                         fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
                 worker.waitForAWhile(shared_from_this());
                 outputLocks.unlock();