Noticed the problem on never disappearing diff reported by:
$ ./maintainers/scripts/rebuild-amount.sh --print HEAD
Estimating rebuild amount by counting changed Hydra jobs.
1 x86_64-linux
basex.x86_64-linux /nix/store/5ng...-basex-9.4.3
Before the change local non-store path was used:
$ nix-build -A basex; fgrep -R Icon result/
result/share/applications/basex.desktop:Icon=/home/.../tools/text/xml/basex/basex.svg
After the change the file got into store as expected:
$ nix-build -A basex; fgrep -R Icon result/
result/share/applications/basex.desktop:Icon=/nix/store/...-basex.svg