Nightly build ref: add --first-parent to midnight commit selection

[sfreudenthaler]

Part of dotCMS/private-issues#673.

.github/workflows/cicd_4-nightly.yml:110 selects the build ref with git log --before=midnight -1. Under merge commits, intermediate branch commits are reachable from main, so the nightly can build a mid-branch tree that never existed as main's tip and never passed the merge queue.

Fix: add --first-parent (both the --before lookup and the fallback at line 115). Safe to land now.

[github-actions]

PRs linked to this issue

• fix(ci): select the nightly build ref along main's first-parent line by @sfreudenthaler