Merge branch '185.use-stable-nix' into 'develop'

Don't require the deprecated nixUnstable package

Closes #185

See merge request !533

ci-tools/update-nixpkgs

 #!/usr/bin/env nix-shell
-#!nix-shell -i bash -p nixUnstable git curl python3
+#!nix-shell -i bash -p git curl python3
 
 # ^^
-# we get nixUnstable for the diff-closures command, mostly.
 # we need git to commit and push our changes
 # we need curl to create the gitlab MR
 # we need python to format the data as json