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.

flake.nix

@@ -101,6 +101,7 @@
             doomDir = ./doomdirs/minimal;
             doomLocalDir = "~/.local/share/nix-doom-unstraightened";
             full = true;
+            experimentalFetchTree = true;
           }).doomEmacs.emacsWithPackages.deps;
         };
       });