Have an option to turn off Vulkan layers

Since v0.20201124.0, pressure-vessel includes Vulkan layers from the host in the container, in addition to Vulkan ICDs.

This is useful functionality, but it comes with a risk. Increasing the number of loadable modules that we load from the host system increases the number of their dependencies that get pulled in, and increasing the number of dependencies that get pulled in reduces the predictability of the runtime environment, re-introducing more of the LD_LIBRARY_PATH runtime's problems.

It would be nice to have an environment variable/command-line option pair (similar to --keep-game-overlay, --remove-game-overlay, PRESSURE_VESSEL_REMOVE_GAME_OVERLAY) that would disable the Vulkan layers, and wire it up to the UI in pressure-vessel/pressure-vessel-test-ui. This would mean that users who are experiencing crashes or other weirdness could toggle that option off and see whether it solves the problem.

Perhaps PRESSURE_VESSEL_ENABLE_VULKAN_LAYERS=[0|1], --enable-vulkan-layers, --disable-vulkan-layers?