4ce1994159
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