Nix: move to devshell

shell.nix

@@ -0,0 +1,23 @@
+# Use `builtins.getFlake` if available
+if builtins ? getFlake
+then
+  let
+    scheme =
+      if builtins.pathExists ./.git
+      then "git+file"
+      else "path";
+  in
+  (builtins.getFlake "${scheme}://${toString ./.}").devShells.${builtins.currentSystem}.default
+
+# Otherwise we'll use the flake-compat shim
+else
+  (import
+    (
+      let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
+      fetchTarball {
+        url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+        sha256 = lock.nodes.flake-compat.locked.narHash;
+      }
+    )
+    { src = ./.; }
+  ).shellNix