refactor: reduce go/build overlay to minimal 87-line patch

Reduced the go/build overlay from 2073 lines to just 87 lines by only
including the patched defaultContext() function and its direct dependencies
(envOr, defaultGOPATH, and related variables).

The overlay system works by merging with the standard library, so we only
need to provide the functions we're modifying. Unpatched functions
automatically fall back to the Go standard library implementation.

This makes the patch much more maintainable and clearly shows what's
being modified: just the Compiler field assignment in defaultContext().

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Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

Author: xgopilot