1ogre:~% p=$(nix-build -E '(import <nixpkgs> {}).writeScriptBin "temprootmagic" "sleep 5"'  --no-out-link)
these derivations will be built:
  /nix/store/yyqa3qpyzlix54sl04l7w5cxl4brn0jy-temprootmagic.drv
building '/nix/store/yyqa3qpyzlix54sl04l7w5cxl4brn0jy-temprootmagic.drv'...

0ogre:~% $p/bin/temprootmagic & nix-store --delete $p; sleep 6; nix-store --delete $p
[1] 28423
finding garbage collector roots...
removing stale link from '/nix/var/nix/gcroots/auto/aq6gwzvblglasi3znn83pf4mj39fmhr3' to '/tmp/nix-build-26603-0/result'
0 store paths deleted, 0.00 MiB freed
error: cannot delete path '/nix/store/jjry942xbkdxbvnnciqz9msbalyrk4wk-temprootmagic' since it is still alive
[1]  + done       $p/bin/temprootmagic
finding garbage collector roots...
deleting '/nix/store/jjry942xbkdxbvnnciqz9msbalyrk4wk-temprootmagic'
deleting '/nix/store/trash'
deleting unused links...
note: currently hard linking saves 103952.45 MiB
1 store paths deleted, 0.00 MiB freed