CI: Fix fetching the latest commit from the backport branch

[leofang]

Turns out #381 was only a partial fix and I should have run the full CI there 😞 This PR replaces it with a better fix to avoid failures seen in the main branch:
https://github.com/NVIDIA/cuda-python/actions/runs/12736445625
where the LATEST_PRIOR_RUN_ID is empty due to the extra quotation added in #381.

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[leofang]

/ok to test

[leofang]

Let me add the backport label and admin-merge to test the backport CI. It should leave a comment in this PR to report an error.

[github-actions]

Backport failed for 11.8.x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 11.8.x
git worktree add -d .worktree/backport-384-to-11.8.x origin/11.8.x
cd .worktree/backport-384-to-11.8.x
git switch --create backport-384-to-11.8.x
git cherry-pick -x aa880355e6d31fa2e7a69e7e15f5561c4417d8a9 9bcf9d665bfc047a7e9a764a74ec2e61d29aa01e

[github-actions]

Backport failed for 11.8.x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 11.8.x
git worktree add -d .worktree/backport-384-to-11.8.x origin/11.8.x
cd .worktree/backport-384-to-11.8.x
git switch --create backport-384-to-11.8.x
git cherry-pick -x aa880355e6d31fa2e7a69e7e15f5561c4417d8a9 9bcf9d665bfc047a7e9a764a74ec2e61d29aa01e

[leofang]

This PR replaces it with a better fix to avoid failures seen in the main branch

this is now fixed