Mike Blumenkrantz replied to me with the correct command line! These environment variables work:
__GLX_VENDOR_LIBRARY_NAME=mesa MESA_LOADER_DRIVER_OVERRIDE=zink GALLIUM_DRIVER=zink
So, this is working here, with my Nvidia GTX 1080 Ti!
[alex@alex-pc ~]$ __GLX_VENDOR_LIBRARY_NAME=mesa MESA_LOADER_DRIVER_OVERRIDE=zink GALLIUM_DRIVER=zink glxinfo | grep OpenGL
skipping config: /usr/bin/MangoHud.conf [ not found ]
skipping config: /home/alex/.config/MangoHud/glxinfo.conf [ not found ]
skipping config: /home/alex/.config/MangoHud/MangoHud.conf [ not found ]
OpenGL vendor string: Collabora Ltd
OpenGL renderer string: zink (NVIDIA GeForce GTX 1080 Ti)
OpenGL core profile version string: 4.6 (Core Profile) Mesa 21.2.1
OpenGL core profile shading language version string: 4.60
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile
OpenGL core profile extensions:
OpenGL version string: 3.0 Mesa 21.2.1
OpenGL shading language version string: 1.30
OpenGL context flags: (none)
OpenGL extensions:
OpenGL ES profile version string: OpenGL ES 3.1 Mesa 21.2.1
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10
OpenGL ES profile extensions:
[alex@alex-pc ~]$