chore(nix): speed up nix-shell usage dramatically

The issue is that any nix flake related command copies the whole
source tree under `/nix/store`  again and again for every file
modification. This behavior needs a proper fix from upstream[^1].

Fortunately it can be worked around by removing unblob build from the
shell environment dependencies, and using the legacy `shell.nix`
entrypoint. Here we can prefilter the source tree before involving
`flake.nix` in the picture. Copying will still happen, but much less
frequently, as source and tests files will be filtered out.

As all our dependencies well behaving now, we can easily use poetry
inside a nix-shell env, no workarounds for build quirks needed.

[^1]: NixOS/nix#3121

flake.nix

@@ -52,9 +52,23 @@
       checks = forAllSystems (system: nixpkgsFor.${system}.unblob.tests);
 
       devShells = forAllSystems
-        (system: {
-          default = import ./shell.nix { pkgs = nixpkgsFor.${system}; };
-        });
+        (system:
+          with nixpkgsFor.${system}; {
+            default = mkShell {
+              packages = [
+                unblob.runtimeDeps
+                ruff
+                pyright
+                python3Packages.pytest
+                python3Packages.pytest-cov
+                poetry
+
+                nvfetcher
+              ];
+
+              env.LD_LIBRARY_PATH = lib.makeLibraryPath [ file ];
+            };
+          });
 
       legacyPackages = forAllSystems (system: nixpkgsFor.${system});
     };

shell.nix

@@ -1,20 +1,24 @@
 # This file is to let "legacy" nix-shell command work in addition to `nix develop`
 let
-  flake = builtins.getFlake (toString ./.);
-  flakePkgs = flake.legacyPackages.${builtins.currentSystem};
-in
-{ pkgs ? flakePkgs }:
+  flakeManifest = [
+    ./flake.lock
+    ./flake.nix
+    ./overlay.nix
+    ./nix
+  ];
 
-with pkgs; mkShell {
-  packages = [
-    unblob
-    unblob.runtimeDeps
-    ruff
-    pyright
-    python3Packages.pytest
-    python3Packages.pytest-cov
-    poetry
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  flake-compat = fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash;
+  };
 
-    nvfetcher
-  ];
-}
+  startsWith = pref: str: with builtins; substring 0 (stringLength pref) str == pref;
+
+  src = builtins.path {
+    path = ./.;
+    name = "source";
+    filter = path: type: builtins.any (x: startsWith (toString x) path) flakeManifest;
+  };
+in
+(import flake-compat { inherit src; }).shellNix.default