update flake.nix and add dependencies as submodules

[chirp] / ext / zighttp

diff --git a/ext/zighttp b/ext/zighttp
new file mode 160000 (submodule)
index 0000000..274c46e
--- /dev/null
+++ b/ext/zighttp
@@ -0,0 +1 @@
+Subproject commit 274c46eb3f7e9987cc9260bb70c43b4f5d8555ac