github actions: handle PRs from forked repos

Detect whether the PR is from a fork and clone from the appropriate
source. Use env vars for GitHub context values to avoid script injection.
Explicitly name the clone destination to avoid directory mismatch if a
fork has a different repo name.

Note: the standard checkout action is not used here because the git
version in this container is too old.

Author: bmastbergen

.github/workflows/build-check_x86_64.yml

@@ -27,8 +27,16 @@ jobs:
           yum install bc dwarves git glibc-devel hostname kernel-devel mpfr openssl openssl-devel elfutils-libelf-devel -y
 
       - name: Checkout code
+        env:
+          HEAD_REF: ${{ github.head_ref }}
+          PR_CLONE_URL: ${{ github.event.pull_request.head.repo.clone_url }}
+          PR_REPO_FULL_NAME: ${{ github.event.pull_request.head.repo.full_name }}
         run: |
-          git clone --branch ${{ github.head_ref }} "https://oauth2:$GITHUB_TOKEN@github.com/ctrliq/kernel-src-tree"
+          if [ "$PR_REPO_FULL_NAME" = "ctrliq/kernel-src-tree" ]; then
+            git clone --branch "$HEAD_REF" "https://oauth2:${GITHUB_TOKEN}@github.com/ctrliq/kernel-src-tree" kernel-src-tree
+          else
+            git clone --branch "$HEAD_REF" "$PR_CLONE_URL" kernel-src-tree
+          fi
 
       - name: Build the Kernel
         working-directory: kernel-src-tree