tests: explicitly define NIX_CONFIG

Ensure NIX_CONFIG is correctly configured for the tests command.

It is possible that someone may enable experimental features on an
ad-hoc basis, e.g. via the `--extra-experimental-features` CLI flag.
In this scenario, the tests script cannot assume they are already
enabled.

In the future, we may also wish to configure other things, like extra
binary substitutors.

tests/package.nix

@@ -10,6 +10,14 @@ writeShellApplication {
     python3
     fzf
   ];
+  runtimeEnv = {
+    # Explicitly enable experimental features, in case someone runs e.g.
+    #   nix run .#tests --extra-experimental-features 'nix-command flakes'
+    # without enabling them globally.
+    NIX_CONFIG = ''
+      experimental-features = nix-command flakes
+    '';
+  };
   text = ''
     exec python3 ${flake}/tests/tests.py "$@"
   '';