For some reason the "Set SteamVR as OpenXR Runtime" button is broken on Linux, here's how to do it manually.
Make an OpenXR config directory if it doesn't exist
$ mkdir -p ~/.config/openxr/1
Link SteamXR to be the default OpenXR runtime (assuming Steam is installed in the default directory)
$ ln -s ~/.local/share/Steam/steamapps/common/SteamVR/steamxr_linux64.json ~/.config/openxr/1/active_runtime.json