Fix handling of child processes on linux

* We need to ensure we remodify LD_LIBRARY_PATH and LD_PRELOAD before fork/exec
  if the application (say bash running a script) has overwritten them. We also
  don't want these to be accidentally inherited into children if we're not
  hooking children - the same for the vulkan env var, which can't be unset
  immediately on process injection like the others because it needs to hang
  around indefinitely.

renderdoc/common/globalconfig.h

@@ -165,6 +165,7 @@ enum
 };
 
 #define RENDERDOC_VULKAN_LAYER_NAME "VK_LAYER_RENDERDOC_Capture"
+#define RENDERDOC_VULKAN_LAYER_VAR "ENABLE_VULKAN_RENDERDOC_CAPTURE"
 
 #define RENDERDOC_ANDROID_LIBRARY "libVkLayer_GLES_RenderDoc.so"