Skip localization PR creation on file-change CI trigger (#40501)

* Skip localization PR creation on file-change CI trigger

The nightly localization pipeline runs both on schedule and when
localization/strings/en-US/Resources.resw is updated. The CI-triggered
run produces a no-op Touchdown pass that still creates a PR, consuming
build resources without updating any strings.

Gate the Create-PR step on Build.Reason in (Schedule, Manual) so the PR
is only created for the nightly schedule (and manual runs), not when
the pipeline is triggered by a push to Resources.resw.

Follow-up to #40499

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Also gate PipAuthenticate on Schedule/Manual

The pip auth step only exists to support create-change.py; if we're not
creating a PR there's no reason to authenticate to the pip feed.

Keep TouchdownBuildTask running on file-change CI so new source strings
are still submitted to the translation service.

Addresses review feedback from @OneBlue.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.pipelines/wsl-build-nightly-localization.yml

@@ -43,6 +43,7 @@ steps:
     cultureMappingType: 'None'
 
 - task: PipAuthenticate@1
+  condition: and(succeeded(), in(variables['Build.Reason'], 'Schedule', 'Manual'))
   inputs:
     artifactFeeds: 'wsl'
 
@@ -51,6 +52,7 @@ steps:
             python tools/devops/create-change.py . "$env:token" "WSL localization"  "Localization change from build: $env:buildId" "user/localization/$env:buildId" "$env:targetBranch"
 
   displayName: Create pull request
+  condition: and(succeeded(), in(variables['Build.Reason'], 'Schedule', 'Manual'))
   env:
       token: $(GithubPRToken)
       buildId: $(Build.BuildId)