ci: update dependabot commit format

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/f5dad40450d272a1ea2413f4a67ac08760649e89' (2023-02-22)
  → 'github:NixOS/nixpkgs/9952d6bc395f5841262b006fbace8dd7e143b634' (2023-02-26)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

.github/dependabot.yml

@@ -4,3 +4,5 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+  - commit-message:
+      prefix: "ci"

flake.lock

@@ -17,11 +17,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1677080879,
-        "narHash": "sha256-0SjW4/d3Rkw6C7hHZ5lxT4r6Pw9vzQb6Il6zYWwe2Bo=",
+        "lastModified": 1677383253,
+        "narHash": "sha256-UfpzWfSxkfXHnb4boXZNaKsAcUrZT9Hw+tao1oZxd08=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "f5dad40450d272a1ea2413f4a67ac08760649e89",
+        "rev": "9952d6bc395f5841262b006fbace8dd7e143b634",
         "type": "github"
       },
       "original": {