Fix fatal "broken pipe" error when $NIX_BUILD_HOOK is missing

src/libstore/build.cc

@@ -1580,6 +1580,8 @@ HookReply DerivationGoal::tryBuildHook()
     if (!worker.hook)
         worker.hook = std::make_unique<HookInstance>();
 
+    try {
+
         /* Tell the hook about system features (beyond the system type)
            required from the build machine.  (The hook could parse the
            drv file itself, but this is easier.) */
@@ -1611,6 +1613,16 @@ HookReply DerivationGoal::tryBuildHook()
         else if (reply != "accept")
             throw Error(format("bad hook reply ‘%1%’") % reply);
 
+    } catch (SysError & e) {
+        if (e.errNo == EPIPE) {
+            printError("build hook died unexpectedly: %s",
+                chomp(drainFD(worker.hook->fromHook.readSide.get())));
+            worker.hook = 0;
+            return rpDecline;
+        } else
+            throw;
+    }
+
     printMsg(lvlTalkative, format("using hook to build path(s) %1%") % showPaths(missingPaths));
 
     hook = std::move(worker.hook);