[cbr79] Fix build check for forked repos

[bmastbergen]

Here is a test PR with the same workflow from a forked repo. It ran the build check just fine:
#961

commit cd6249ce75127adf489d97ebe15ee952a77aeb56
Author: Brett Mastbergen <bmastbergen@ciq.com>
Date:   Wed Mar 11 12:08:22 2026 -0400

    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.

 .github/workflows/build-check_x86_64.yml | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

commit dc93d458821e12022e69c5d6100142fd97621445
Author: Brett Mastbergen <bmastbergen@ciq.com>
Date:   Wed Mar 11 12:21:32 2026 -0400

    github actions: use shallow clone for kernel checkout

 .github/workflows/build-check_x86_64.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

[github-actions]

🤖 Validation Checks In Progress Workflow run: https://github.com/ctrliq/kernel-src-tree/actions/runs/22964980625

[github-actions]

✅ Validation checks completed successfully View full results: https://github.com/ctrliq/kernel-src-tree/actions/runs/22964980625