ci: remove redundant macos runner for CI tests

Currently the macOS runner is not needed, as we are only checking
zsh syntax. We can reactivate it in the future if needed.

.github/workflows/main.yml

@@ -20,16 +20,12 @@ permissions:
 jobs:
   tests:
     name: Run tests
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     if: github.repository == 'ohmyzsh/ohmyzsh'
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest]
     steps:
       - name: Set up git repository
         uses: actions/checkout@v3
       - name: Install zsh
-        if: runner.os == 'Linux'
         run: sudo apt-get update; sudo apt-get install zsh
       - name: Check syntax
         run: |