Add `dch` and `notify-send` to the development Nix flake so that the release script can be used. (#15673)

* Add dch and notify-send to the Nix dev flake

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

---------

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

changelog.d/15673.misc

@@ -0,0 +1 @@
+Add `dch` and `notify-send` to the development Nix flake so that the release script can be used.

flake.nix

@@ -100,6 +100,10 @@
 
                   # For building the Synapse documentation website.
                   mdbook
+
+                  # For releasing Synapse
+                  debian-devscripts # (`dch` for manipulating the Debian changelog)
+                  libnotify # (the release script uses `notify-send` to tell you when CI jobs are done)
                 ];
 
                 # Install Python and manage a virtualenv with Poetry.