Fix Cloning in CI for Forks (#249)

Fix that manually retrieving the code in the gcc-6 action stage does not work for PRs from forks.

.github/workflows/ci.yml

@@ -55,11 +55,15 @@ jobs:
         if: matrix.name == 'gcc-6'
         # Recent versions of Github's checkout action do not run on older Ubuntu versions because they use a too recent
         # Node.js version. Thus, we have to checkout the code manually.
+        # Doing so is a bit tricky when it comes to PRs from forks (see #249 for details). The general idea here is that
+        # we access Github's context information for the event triggering the action's execution and use some details on
+        # the PR's HEAD if given. Otherwise (for executions due to master updates), we still use the provided
+        # environment variables.
         run: |
           apt-get update
           apt-get install -y git
           git config --global --add safe.directory '*'
-          git clone "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}" .
+          git clone $(awk -v a=${{github.event.pull_request.head.repo.clone_url}} -v b="${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}" 'BEGIN { if (a == "")  { print b } else { print a } }') .
           git checkout $GITHUB_HEAD_REF
 
       - name: Setup (macOS)