Pipeline: fix loc/notice PR base branch for non-master runs (#40510)
Both wsl-build-nightly-localization.yml and wsl-build-notice.yml derived the PR target branch from Build.SourceBranchName, which Azure DevOps documents as the leaf segment after the last '/' in the source ref. For refs/heads/master that happens to round-trip ('master'), so scheduled runs work; but a manual queue against any branch with slashes (e.g. user/benhill/loc-pipeline-debug) sends only the leaf ('loc-pipeline-debug') as the PR base, which GitHub rejects with 422 Validation Failed: base invalid. This is the same bug PR #14492 introduced when it parameterized the target branch -- the truncation case was never exercised because the real schedule only runs on master. Use Build.SourceBranch and strip the 'refs/heads/' prefix at runtime so the full branch name is preserved. master nightlies are unaffected ('refs/heads/master' -> 'master'). Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>