Prevent a deadlock when user namespace setup fails

Observed on Centos 7 when user namespaces are disabled:
DerivationGoal::startBuilder() throws an exception, ~DerivationGoal()
waits for the child process to exit, but the child process hangs
forever in drainFD(userNamespaceSync.readSide.get()) in
DerivationGoal::runChild(). Not sure why the SIGKILL doesn't get
through.

Issue #4092.

(cherry picked from commit d761485010)

src/libstore/build.cc

@@ -2388,6 +2388,12 @@ void DerivationGoal::startBuilder()
 
         userNamespaceSync.readSide = -1;
 
+        /* Close the write side to prevent runChild() from hanging
+           reading from this. */
+        Finally cleanup([&]() {
+            userNamespaceSync.writeSide = -1;
+        });
+
         pid_t tmp;
         if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort();
         pid = tmp;
@@ -2409,7 +2415,6 @@ void DerivationGoal::startBuilder()
         /* Signal the builder that we've updated its user
            namespace. */
         writeFull(userNamespaceSync.writeSide.get(), "1");
-        userNamespaceSync.writeSide = -1;
 
     } else
 #endif