ci: restrict tests job to run only on pull requests (#2705)

The 'tests' job relies on the GitHub CLI (`gh`) to parse the commits and
description of an associated pull request to find triggers. When running
on a standard branch push (such as to 'main'), there is no pull request
context, making it impossible to determine which commits to parse.

Also add missing if on steps to make sure nothing runs when
appropriate.

.github/workflows/presubmit.yml

@@ -182,6 +182,7 @@ jobs:
   tests:
     name: Run Tests (pocl)
     runs-on: ubuntu-24.04
+    if: github.event_name == 'pull_request'
     steps:
       - uses: actions/checkout@v6
       - name: Parse triggers
@@ -214,8 +215,10 @@ jobs:
           git clone --depth 1 https://github.com/KhronosGroup/OpenCL-Headers.git ${{ env.cl-headers }}
           git clone --depth 1 https://github.com/KhronosGroup/SPIRV-Headers.git ${{ env.spv-headers }}
       - name: Setup Ninja
+        if: steps.check_triggers.outputs.run_tests == 'true'
         uses: seanmiddleditch/gha-setup-ninja@master
       - name: Install Vulkan SDK
+        if: steps.check_triggers.outputs.run_tests == 'true'
         uses: humbletim/install-vulkan-sdk@main
         with:
           version: 1.4.309.0