[skip ci] Skip push to forks

Creating PRs against forks requries having up-to-date target branches. Pushing
to these currently triggers CI, which is useless as this commit was already
tested upstream. Contributions are tested via pull request.

.github/workflows/push.yml

@@ -43,6 +43,7 @@ env:
   CXX: ccache g++
 jobs:
   LINUX_X64:
+    if: github.repository_owner == 'php' || github.event_name == 'pull_request'
     services:
       mysql:
         image: mysql:8
@@ -148,6 +149,7 @@ jobs:
         if: ${{ !matrix.asan }}
         uses: ./.github/actions/verify-generated-files
   MACOS_DEBUG_NTS:
+    if: github.repository_owner == 'php' || github.event_name == 'pull_request'
     strategy:
       fail-fast: false
       matrix:
@@ -189,6 +191,7 @@ jobs:
       - name: Verify generated files are up to date
         uses: ./.github/actions/verify-generated-files
   WINDOWS:
+    if: github.repository_owner == 'php' || github.event_name == 'pull_request'
     name: WINDOWS_X64_ZTS
     runs-on: windows-2019
     env: