Restore the weekly schedule to Build Loop and phase out Build Loop Auto - #581

Summary

• Move the weekly schedule back to build_loop.yml and rename it from "4. Build Loop Manual" to "4. Build Loop"
• Turn build_loop_auto.yml into "zzz Phasing Out": no schedule, manual run only. It will be removed from the repo in a future release
• Skip the fastlane update check in all workflows that run fastlane (borrowed from Skip the fastlane update check in the build workflow loopandlearn/LoopFollow#828)

Why

GitHub keeps disabling the scheduled Build Loop workflow in people's forks, so the weekly sync and build stop running. We never found the cause. A workflow with a new file name (build_loop_auto.yml) worked while only a few people used it. After release it began being disabled the same way the original build_loop.yml had been.

Since a new file name didn't help, this PR puts the schedule back in build_loop.yml, so there is one build workflow that runs both manually and on schedule. The "cancel when there is nothing to build" job moves with it, so a scheduled run still shows as cancelled, not successful, when no build was made.

build_loop_auto.yml is kept for now because some people have customizations in it. They can copy them into build_loop.yml before the file is removed. The "zzz" prefix puts it at the bottom of the Actions list.

Fastlane update check

fastlane is pinned in the Gemfile, but when a newer release exists it still prints "Please update using bundle update fastlane" and the full changelog at the end of every run. When a build fails, that notice comes after the real error and is easy to mistake for the cause. FASTLANE_SKIP_UPDATE_CHECK: true is now set in Validate Secrets, Add Identifiers, Create Certificates, Build Loop, and the phasing-out workflow. The pin is still bumped by hand when a newer fastlane is needed.

What users need to do after updating

• If "4. Build Loop" was disabled by GitHub in the past, enable it in the Actions tab. Otherwise the schedule won't run
• If you customized build_loop_auto.yml, move those changes into build_loop.yml
• Use "4. Build Loop" for manual builds. "zzz Phasing Out" is there only for the transition

Follow-up

• Update LoopDocs that refer to "Build Loop Manual" or "Build Loop Auto"
• Remove build_loop_auto.yml in a later release

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