Stop running pull request workflow on main

Instead of running the checks on `main`, I've opted to just rebase the current branch onto `main` as part of the workflow process so that I don't need to run checks twice, which starves CI resources and doubles the minutes I'd run tests on Firebase Test Lab

.forgejo/workflows/pull_request.yml

@@ -2,8 +2,6 @@ name: Build & Test
 
 on:
   pull_request:
-  push:
-    branches: [ main ]
 
 jobs:
   validate:
@@ -11,6 +9,7 @@ jobs:
     name: Validate
     steps:
     - uses: actions/checkout@v4
+    - run: git rebase origin/main
     - name: set up JDK
       uses: https://git.wbrawner.com/actions/setup-java@v4
       with: