Binary symbol-server lookup 404s for /Brepro builds with leading-zero TimeDateStamp

[ivberg]

• SymbolReader.cs:525 does buildTimestamp.ToString("x"); the symbol-server convention (matched by symsrv.dll/symchk) is TimeDateStamp:X8 + SizeOfImage:X.
• Affects PE binaries whose TimeDateStamp's canonical form has a leading zero (i.e., (uint)TS < 0x10000000).
• Real-world repro: any binary built with /Brepro whose deterministic-hash TimeDateStamp happens to start with a zero nibble. Concrete example: SystemSettings.dll on Windows 11 with TS=0x0D9F641E, SizeOfImage=0x599000 — server has 0d9f641e599000, TraceEvent emits d9f641e599000 and 404s.
• Fix: buildTimestamp.ToString("x") → buildTimestamp.ToString("x8"). SizeOfImage is correctly variable-width and shouldn't change.

Does it also affect PDB download? No.

PDB lookup (FindSymbolFilePath) uses a different key: Guid:NAge:X.

• The GUID is fixed 32 hex chars regardless of value (Guid.ToString("N") always emits 32) — no padding issue there.
• Age is variable-width by convention: both symsrv and TraceEvent agree on no-padding (e.g., age=1 → …1, age=16 → …10). So they match.