Use minimal_symbols on debug_bot in mb_config.pyl

debug_bot configs defaulted to symbol_level=2 (full PDBs on Windows),
causing severe disk I/O thrashing and multi-hour link phases on Windows
debug builders such as win_x86_more_configs.

Include minimal_symbols (symbol_level=1, matching debug_static_bot) in
the debug_bot mixin so crash backtraces still have function and line
info without generating full type/variable debug symbols on bots.

Symbols with internal linkage (static functions or those in anonymous
namespaces) may not appear when using this level.

Bug: webrtc:498394143
Change-Id: I8ff14b77375ac428d79a2c65a656d59e52e073fe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508320
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Commit-Queue: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48838}