Use fetchTree / github fetcher for CI

This seems to be much more space-efficient: ~/.cache/nix/tarball-cache
is about 700MiB uncompressed, under 300MiB as tzst (using tar's
defaults, CI uses zstdmt but I assume will be in the same ballpark). The
gitv3 cache is multiple GiB.

CI will still build doom-example using fetchGit. I intend to shrink the
number of modules enabled in the example to keep gitv3 cache size under
control.

checks.nix

@@ -31,6 +31,7 @@ let
     # TODO: drop after NixOS 24.05 release.
     emacs = emacs29;
     doomLocalDir = "~/.local/share/nix-doom-unstraightened";
+    experimentalFetchTree = true;
   };
   mkDoom = args: (makeDoomPackages (common // args)).doomEmacs;
   mkDoomDir = args: writeTextDir "init.el" (toInit args);