feat(flake): remove impurity, update nix-direnv

Use path as an input that can be overriden automatically when direnv is
in use.
nix-direnv version present in .envrc has been updated, using watch_file on the flake is already handled.

flake.lock

@@ -39,11 +39,11 @@
         "pre-commit-hooks": "pre-commit-hooks"
       },
       "locked": {
-        "lastModified": 1724504184,
-        "narHash": "sha256-gP6000c2+zHKJHAxCD3BftvAjmb4CPAZamRAHNxN2MM=",
+        "lastModified": 1724763216,
+        "narHash": "sha256-oW2bwCrJpIzibCNK6zfIDaIQw765yMAuMSG2gyZfGv0=",
         "owner": "cachix",
         "repo": "devenv",
-        "rev": "51338b58fd666f448db7486ec145dbe52db9b829",
+        "rev": "1e4ef61205b9aa20fe04bf1c468b6a316281c4f1",
         "type": "github"
       },
       "original": {
@@ -52,6 +52,18 @@
         "type": "github"
       }
     },
+    "devenv-root": {
+      "flake": false,
+      "locked": {
+        "narHash": "sha256-d6xi4mKdjkX2JFicDIv5niSzpyI0m/Hnm8GGAIU04kY=",
+        "type": "file",
+        "url": "file:///dev/null"
+      },
+      "original": {
+        "type": "file",
+        "url": "file:///dev/null"
+      }
+    },
     "devenv_2": {
       "inputs": {
         "flake-compat": [
@@ -122,11 +134,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1722555600,
-        "narHash": "sha256-XOQkdLafnb/p9ij77byFQjDf5m5QYl9b2REiVClC+x4=",
+        "lastModified": 1725024810,
+        "narHash": "sha256-ODYRm8zHfLTH3soTFWE452ydPYz2iTvr9T8ftDMUQ3E=",
         "owner": "hercules-ci",
         "repo": "flake-parts",
-        "rev": "8471fe90ad337a8074e957b69ca4d0089218391d",
+        "rev": "af510d4a62d071ea13925ce41c95e3dec816c01d",
         "type": "github"
       },
       "original": {
@@ -269,11 +281,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1720642556,
-        "narHash": "sha256-qsnqk13UmREKmRT7c8hEnz26X3GFFyIQrqx4EaRc1Is=",
+        "lastModified": 1724996935,
+        "narHash": "sha256-njRK9vvZ1JJsP8oV2OgkBrpJhgQezI03S7gzskCcHos=",
         "owner": "nlewo",
         "repo": "nix2container",
-        "rev": "3853e5caf9ad24103b13aa6e0e8bcebb47649fe4",
+        "rev": "fa6bb0a1159f55d071ba99331355955ae30b3401",
         "type": "github"
       },
       "original": {
@@ -407,11 +419,11 @@
     },
     "nixpkgs_3": {
       "locked": {
-        "lastModified": 1724224976,
-        "narHash": "sha256-Z/ELQhrSd7bMzTO8r7NZgi9g5emh+aRKoCdaAv5fiO0=",
+        "lastModified": 1724819573,
+        "narHash": "sha256-GnR7/ibgIH1vhoy8cYdmXE6iyZqKqFxQSVkFgosBh6w=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "c374d94f1536013ca8e92341b540eba4c22f9c62",
+        "rev": "71e91c409d1e654808b2621f28a327acfdad8dc2",
         "type": "github"
       },
       "original": {
@@ -477,6 +489,7 @@
     "root": {
       "inputs": {
         "devenv": "devenv",
+        "devenv-root": "devenv-root",
         "flake-parts": "flake-parts",
         "nix2container": "nix2container",
         "nixpkgs": "nixpkgs_3",