Don't delete .check directories of running builds

We need to keep them around for diffoscope.

src/libstore/build.cc

@@ -2687,7 +2687,7 @@ void DerivationGoal::registerOutputs()
        outputs to allow hard links between outputs. */
     InodesSeen inodesSeen;
 
-    Path checkSuffix = "-check";
+    Path checkSuffix = ".check";
     bool runDiffHook = settings.get("run-diff-hook", false);
     bool keepPreviousRound = settings.keepFailed || runDiffHook;
 
@@ -2962,7 +2962,7 @@ void DerivationGoal::registerOutputs()
         return;
     }
 
-    /* Remove the -check directories if we're done. FIXME: keep them
+    /* Remove the .check directories if we're done. FIXME: keep them
        if the result was not determistic? */
     if (curRound == nrRounds) {
         for (auto & i : drv->outputs) {

src/libstore/gc.cc

@@ -621,6 +621,11 @@ void LocalStore::tryToDelete(GCState & state, const Path & path)
         /* Don't delete .chroot directories for derivations that are
            currently being built. */
         if (isActiveTempFile(state, path, ".chroot")) return;
+
+        /* Don't delete .check directories for derivations that are
+           currently being built, because we may need to run
+           diff-hook. */
+        if (isActiveTempFile(state, path, ".check")) return;
     }
 
     PathSet visited;