Build(flake): Enable direnv integration

[Direnv](1) in combination with some sort of [Nix integration](2)
— in this case [Nix-direnv](3) — allows for reproducible development
environments (and allows uncluttering the default PATH, as cargo and
other development tools are no longer needed in it).

Setting it up is rather easy, just see [Nix-direnv's install
instructions](4).

[1]: https://github.com/direnv/direnv
[2]: https://github.com/direnv/direnv/wiki/Nix
[3]: https://github.com/nix-community/nix-direnv
[4]: https://github.com/nix-community/nix-direnv#installation

.envrc

@@ -0,0 +1,7 @@
+use flake || use nix
+watch_file flake.nix
+
+if on_git_branch; then
+  echo && git status --short --branch &&
+  echo && git fetch --verbose
+fi

.gitignore

@@ -1,6 +1,10 @@
-target/
 userdata/
 trinitrix.log
 
+# build
+/target
+/result
+
 # IDE stuff
 .idea
+.direnv