Respect max-substitution-jobs again

This broke in NixOS#11005. Any number of PathSubstitutionGoals would
be woken up by a single build slot becoming available. If there
are a lot of substitution goals active, this could lead to us
running out of file descriptors (especially on macOS where the
default limit is 256).

src/libstore/build/substitution-goal.cc

@@ -183,7 +183,7 @@ Goal::Co PathSubstitutionGoal::tryToRun(StorePath subPath, nix::ref<Store> sub,
     /* Make sure that we are allowed to start a substitution.  Note that even
        if maxSubstitutionJobs == 0, we still allow a substituter to run. This
        prevents infinite waiting. */
-    if (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
+    while (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
         worker.waitForBuildSlot(shared_from_this());
         co_await Suspend{};
     }