ci: migrate to 'exec' runner

Now that I have written a script to do the Matrix notification, I can do
that.

.drone.yml

@@ -1,24 +1,27 @@
+---
 kind: pipeline
-name: check config
+type: exec
+name: NixOS config check
 
 steps:
 - name: format check
-    image: nixos/nix
   commands:
-      - nix-shell -p nixpkgs-fmt --run 'nixpkgs-fmt . --check'
+  - nix develop -c nixpkgs-fmt .
 
-  - name: notify
+- name: notifiy
-    image: plugins/matrix
+  commands:
-    settings:
+  - nix run .#matrix-notifier
-      homeserver:
+  environment:
+    ADDRESS:
       from_secret: matrix_homeserver
-      roomid:
+    ROOM:
       from_secret: matrix_roomid
-      username:
+    USER:
       from_secret: matrix_username
-      password:
+    PASS:
       from_secret: matrix_password
-      trigger:
+  when:
     status:
       - failure
       - success
+...