Have Playground blueprint default to main. (aspirepress#173)

* Remove `main` from the Playground workflow.

* Set the Playground blueprint to `main` by default.

* Only run the workflow job on the main repository.

.github/workflows/playground-blueprint.yml

@@ -3,13 +3,13 @@ name: Update Playground Blueprint with Repository and Branch
 on:
   push:
     branches:
-      - main
       - playground-ready
     tags:
       - "**"
 
 jobs:
   update-blueprint:
+    if: github.repository == 'aspirepress/AspireUpdate'
     runs-on: ubuntu-latest
     permissions:
       contents: write