How to override mesa VULKAN_DRIVERS and GALLIUMDRIVERS variables?

I'd like to change the value of these.

However, they are set like this:
VULKAN_DRIVERS = ""
VULKAN_DRIVERS:append:x86 = ",intel,amd"
VULKAN_DRIVERS:append:x86-64 = ",intel,amd"

...
VULKAN_DRIVERS:append ="${@bb.utils.contains('PACKAGECONFIG', 'freedreno', ',freedreno', '', d)}"

Setting VULKAN_DRIVERS in an bbappend or machine conf file to something like "intel" doesn't work, because the append actions still append to that value all additional drivers.
Trying to remove other drivers with something like VULKAN_DRIVERS:remove = ",amd" doesn't work either, as remove only matches on word boundaries and the list is command delimited, not space delimited.