There is no cmake option to change output file names and that’s not how it’s usually done.
Common practice is to pass CMAKE_INSTALL_PREFIX and have your custom build live in it’s own directory.
I was hoping to get advice on something like how to get a custom naming like kicad_nigthly vs kicad, but nevertheless, thanks people, will do an install to a separate directory!
for that to appear in the XDG applications menu, that should be a simple case of applying a patch pre-compile to rename the kicad.application file and its content
I recommend you take a look at the excellent AUR package kicad-nightly that does this exactly, renaming kicad to kicad-nightly. If you aren’t already aware of how the AUR works, you’ll have to git clone https://aur.archlinux.org/kicad-nightly.git, to view the package sources, and you can look at the PKGBUILD file, which is just a shell script, for how it was done.