We need better test coverage to figure out how many more of these there
are. I don't see a way of dealing with them better, but (unless they are
revived) they should not change underneath us, so keeping these
overrides does not seem too risky.
Fix formatting for the sly-stepper pin while I'm there.
CI fails to fetch it:
```
error: Server does not allow request for unadvertised object 87d0d1fb0566a80229029d0d8d7c863138d70aae
warning: could not update mtime for file '/home/runner/.cache/nix/gitv3/0gjpwip102kwcvz961gsiva3lqmmr6266s5wzs8kq0ybm68gwpx9/refs/heads/master': No such file or directory
error:
… while checking flake output 'checks'
at /nix/store/fwrwzxjvvpx1l27h8j5f9gffzwn2vdik-source/flake.nix:54:7:
53| in {
54| checks = perSystemPackages (pkgs:
| ^
55| let
… while checking the derivation 'checks.x86_64-linux.full'
at /nix/store/fwrwzxjvvpx1l27h8j5f9gffzwn2vdik-source/flake.nix:83:11:
82| })).emacsWithDoom;
83| full = mkDoom {
| ^
84| full = true;
(stack trace truncated; use '--show-trace' to show the full trace)
error: Cannot find Git revision '87d0d1fb0566a80229029d0d8d7c863138d70aae' in ref 'refs/heads/master' of repository 'https://git.savannah.gnu.org/git/emms.git'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.
```
Try to follow the suggestions from the error message, as I do see the
commit in https://git.savannah.gnu.org/cgit/emms.git/.
I'm not sure why this is only failing now...
