If you click the "Latest Switch Builds" link in the OP, you'll find the nightlies. They are potentially the best/fastest/most optimized, but also potentially have the most bugs. Use at your very own risk.
Here's the direct link to the build I referenced:
http://buildbot.libretro.com/nightly/nintendo/switch/libnx/2019-01-14_RetroArch.7z
After you unzip it, just grab the mupen64 nro from the retroarch > cores folder and replace it in the retroarch > cores folder on your SD card. I like to rename the previous build to something like "mupen64plus_libretro_libnx.nro.OLD" just in case the nightly is completely broken. That way I can easily revert back to a working core.
Here's the link to all of the Switch nightlies:
http://buildbot.libretro.com/nightly/nintendo/switch/libnx/
The "latest" folder is where you'll get the bleeding edge stuff. I like to live dangerously so I've been grabbing the latest mupen64 core every night.