You could add the source code of fluxer as a flake input (with flake=false) Then pass that down to the derivation with src=inputs.fluxer-src-name-you-chose.

Then everytime you update that flake input you get the latest version at that point in time. I did something similar here: pdfcat-nix (also works if you use a flake as input with your package in it, but overwrite the flake input to use the latest version without updating the flake lock of the input flake)

You can also check out the way I did automatic version generation based on last commit date and commit hash in bleeding-edge.nix