nix: use devenv to create a shell environment

Devenv[^1] is tool to create easily composable development
environments for nix.

Hooking it in to our nix flake gives us the following improvements:
 - more declarative python environment config
 - avoid the `patchElf` hack to pull in native dependencies
 - python packages updates won't undo the effect of `patchElf`
 - due to a long-standing bug[^2], a patched `devenv` version is needed

[^1]: https://devenv.sh/
[^2]: cachix/devenv-nixpkgs#2

.github/workflows/build-nix.yml

@@ -41,4 +41,4 @@ jobs:
           authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
           extraPullNames: pyperscan
       - run: |
-          nix flake check -L --option system ${{ matrix.arch }} --extra-platforms ${{ matrix.arch }}
+          nix flake check -L --impure --option system ${{ matrix.arch }} --extra-platforms ${{ matrix.arch }}

devenv.nix

@@ -0,0 +1,24 @@
+{ pkgs, ... }:
+
+# https://devenv.sh/reference/options/
+{
+  languages.python = {
+    enable = true;
+    libraries = with pkgs; [
+      file # python-magic
+    ];
+    poetry = {
+      enable = true;
+      activate.enable = true;
+      install.enable = true;
+      install = {
+        groups = [ "dev" ];
+        installRootPackage = true;
+      };
+    };
+  };
+
+  packages = with pkgs; [
+    nvfetcher
+  ] ++ unblob.runtimeDeps;
+}