Combining Stable and Unstable with Home Manager in a Flake

While setting up a new workstation I noticed a few core apps (the Swiss Proton suite of apps) are running behind in updates quite a bit. Since the new versions have both features and security patches I went looking for a way to get them from the unstable nixpkgs instead. The ones I’m trying this with are Electron-wrapped apps making me fairly confident mixing them with stable won’t do any harm. I found this thread and have been trying to implement @waffle8946’s answer into my flake with the pac...