add flake support

This change is fully backwards compatible.

We continue to use the standard non-flake evaluation mode in our
examples and internal tooling until the flakes design has stabilized.

'clightning-plugins = pkgs.recurseIntoAttrs' in pkgs/default.nix is
needed by flake-utils.lib.flattenTree in flake.nix.
It transforms the packages in `clightning-plugins` to top-level packages
named like `clightning-plugins/summary`. (The flake attr `packages`
must be a non-nested attrset of derivations.)

test/run-tests.sh

@@ -281,9 +281,18 @@ examples() {
     (cd "$scriptDir/../examples" && nix-shell --run "$script")
 }
 
+flake() {
+    if [[ $(nix flake 2>&1) != *"requires a sub-command"* ]]; then
+        echo "Skipping flake test. Nix flake support is not enabled."
+    else
+        nix flake check "$scriptDir/.."
+    fi
+}
+
 all() {
     buildable
     examples
+    flake
 }
 
 # An alias for buildTest