libstore: correctly initialize build request union members

oops. we have to use init* here, not get*. get returns a discarding
builder for something that was not inited previously, but the linux
cause worked anyway because the first member is created by default!

Change-Id: I40f8a12a04eef2f4e3a80d1537ac9b975490a027

lix/libstore/platform/darwin.cc

@@ -252,7 +252,7 @@ try {
 
 void DarwinLocalDerivationGoal::fillBuilderConfig(build::Request::Builder request)
 {
-    auto config = request.getPlatform().getDarwin();
+    auto config = request.getPlatform().initDarwin();
 
     /* This has to appear before import statements. */
     std::string sandboxProfile = "(version 1)\n";

lix/libstore/platform/linux.cc

@@ -1248,7 +1248,7 @@ std::string LinuxLocalDerivationGoal::rewriteResolvConf(std::string fromHost)
 
 void LinuxLocalDerivationGoal::fillBuilderConfig(build::Request::Builder request)
 {
-    auto config = request.getPlatform().getLinux();
+    auto config = request.getPlatform().initLinux();
 
 #if HAVE_SECCOMP
     {