Don’t rely on EPERM

startProcess does not appear to send the exit code to the helper
correctly. Not sure why this is, but it is probably safe to just
fallback on all sandbox errors.

src/libstore/build.cc

@@ -2320,15 +2320,15 @@ void DerivationGoal::startBuilder()
                parent. This is only done when sandbox-fallback is set
                to true (the default). */
             if (child == -1 && (errno == EPERM || errno == EINVAL) && settings.sandboxFallback)
-                _exit(EPERM);
+                _exit(1);
             if (child == -1) throw SysError("cloning builder process");
 
             writeFull(builderOut.writeSide.get(), std::to_string(child) + "\n");
             _exit(0);
         }, options);
 
         int res = helper.wait();
-        if (res == EPERM && settings.sandboxFallback) {
+        if (res != 0 && settings.sandboxFallback) {
             useChroot = false;
             goto fallback;
         } else if (res != 0)