Pipelines: fix nuget stage CodeSign failure on in-repo .ps1 scripts (#40685)
The release build's nuget stage (Publish nuget packages) was failing in the Guardian CodeSign post-analysis with 10 CodeSign.MissingSigningCert errors on unsigned dev/diagnostic/test helper .ps1 scripts in the source tree. The OneBranch per-stage SDL CodeSign validation is job-scoped: the pipeline-level ob_sdl_codeSignValidation_excludes variable does not flow into the auto-injected per-stage analysis, so it never had any effect. The stages that pass do so because of the binaries-only globalSdl.codesign.targetGlob (release) or job-level excludes (build-job.yml, package-stage.yml). The nuget job had neither, so its CodeSign validation scanned the checked-out repo and broke on the .ps1 files. Set ob_sdl_codeSignValidation_excludes on the nuget job (mirrors build-job.yml) so the .ps1 scripts are skipped while the published .nupkg files stay validated. Also remove the ineffective pipeline-level ob_sdl_codeSignValidation_excludes variable from the release/nightly/PR OneBranch pipelines (added in #40541): it never suppressed the per-stage CodeSign analysis. The release pipeline is already covered by the binaries-only targetGlob plus per-job excludes, and the nightly/PR pipelines use the NonOfficial template with no breaking codesign. Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>