ci/update: make steps optional

Allow toggling off the flake.lock or generate files steps.

.github/workflows/update.yml

@@ -6,6 +6,14 @@ on:
   # Allow manual triggering
   workflow_dispatch:
     inputs:
+      lock:
+        type: boolean
+        default: true
+        description: Update flake.lock
+      generate:
+        type: boolean
+        default: true
+        description: Update generated files
       nixos-24.05:
         type: boolean
         description: Also update nixos-24.05
@@ -54,6 +62,7 @@ jobs:
 
       - name: Update flake.lock
         id: flake_lock
+        if: inputs.lock || github.event_name == 'schedule'
         run: |
           old=$(git show --no-patch --format=%h)
           nix flake update --commit-lock-file
@@ -66,6 +75,7 @@ jobs:
 
       - name: Update generated files
         id: generate
+        if: inputs.generate || github.event_name == 'schedule'
         run: |
           old=$(git show --no-patch --format=%h)
           nix-build ./update-scripts -A generate