home/nix-direnv: use unreleased version from flake

necessary to get integrated [GC root
refreshing](https://github.com/nix-community/nix-direnv/pull/631).

It currently looks like nix-direnv is going to be integrated into direnv
itself, so maybe nor new releases will be made until then.

flake.lock

@@ -424,11 +424,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1769353635,
-        "narHash": "sha256-J0G1ACrUK29M0THPAsz429eZX07GmR9Bs/b0pB3N0dQ=",
+        "lastModified": 1762938485,
+        "narHash": "sha256-AlEObg0syDl+Spi4LsZIBrjw+snSVU4T8MOeuZJUJjM=",
         "owner": "numtide",
         "repo": "treefmt-nix",
-        "rev": "f46bb205f239b415309f58166f8df6919fa88377",
+        "rev": "5b4ee75aeefd1e2d5a1cc43cf6ba65eba75e83e4",
         "type": "github"
       },
       "original": {