Fix race condition while initializing shader cache

Fix race condition while initializing shader cache

No test results should be affected by this change.

Without this patch, I can often (not always) reproduce a crash when
running vk-build-programs. The crash happens during the first seconds of
the program running, while initializing shader cache variables and using
them for the first time.

With this patch, I can no longer make vk-build-programs crash.

Components: Vulkan
VK-GL-CTS issue: 4269

Change-Id: I61db5e9869849813e9bb451ddd4126ca091077da