bump pixi version in readme every three months

.github/workflows/build.yml

@@ -7,7 +7,9 @@ on:
     branches: [main]
   merge_group:
   schedule:
-    - cron: 0 6 * * MON
+    # every 3 months
+    - cron: 0 0 1 */3 *
+  workflow_dispatch:
 
 jobs:
   build:
@@ -54,6 +56,7 @@ jobs:
 
   reference-latest-pixi-version-in-readme:
     name: Reference latest Pixi version in README
+    if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     permissions:
       contents: write