After set_mpvkit_revision.sh moves the MPVKit pin, macOS builds die in the
SwiftPM integration migration with "could not find the commit <sha>": Flutter
runs the macOS Xcode steps with -skipPackageUpdates, and its one fetch-capable
step only ever runs for iOS, so the local mirror never learns the new revision.
scripts/refresh_apple_spm.sh runs a single -resolvePackageDependencies per
platform to update the mirror without touching the tracked locks, with --reset
for an internally inconsistent SourcePackages directory. The header of
set_mpvkit_revision.sh now points at it.
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