Windows PowerShell 5.1 emits ParserError text in the console ANSI code page,
but the GUI bootstrap aborted BufReader::lines() on the first non-UTF-8 byte
and Retry kept reusing a poisoned install-main.ps1 for branch pins. Decode
child output with a real Windows-1252 fallback, write a UTF-8 BOM on cached
.ps1 files for -File, and refresh mutable branch/tag caches on each run
(immutable SHAs stay cached).
Fixes#67193