Review findings on the local build scripts:
- Cleanup ran before desktop.py took the checkout lock, so a second run deleted
the outputs of a build that was still going. desktop.py gains --clean, which
removes this checkout's outputs after build_lock is held. Both scripts pass it
instead of deleting files themselves.
- build-bundle.ps1 left RELEASE_CLAIM_* and HERMES_PAYLOAD_* in the caller's
session. It now saves those variables and restores them in the finally block.
- The scripts said "unsigned" but inherited signing credentials. The ps1 removes
AZURE_* for the build. The sh removes CSC_* and APPLE_* and sets
CSC_IDENTITY_AUTO_DISCOVERY=false, the switch the desktop rebuild command uses.
The wording now says what is ignored.
- The Store bundle step needs the claim timestamp, and the script deletes the
claim. The ps1 prints it as HERMES_RELEASE_EPOCH, and BUILDING.md documents it.
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