Meson devenv is a feature added in meson 0.58 (thus the features is
version guarded) that allows creating a shell environment with
environment variables automatically setup for running the project inside
the build dir. Some variables (such as LD_LIBRARY_PATH and PATH) are set
automatically, others must be added by the project.

For vulkan is is relativley simple, we create a new, uninstalled, icd
file for each driver and set the VK_ICD_FILENAMES variable
appropriately. This can be used with:

```sh
meson devenv -C $builddir
```

then, vulkan applications will automatically use the uninstall vulkan
driver, no need to install.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <mesa/mesa!14826>