Use selected module for dependency downloads

docs/internal/gotool.md

@@ -33,7 +33,9 @@ coexist. Disk-backed diagnostics could reduce large failure peaks, but add file
 lifecycle and cleanup work and remain a follow-up option.
 
 Dependency preparation uses a separate command environment so proxy, private
-module, checksum, and cache settings remain available. Its completed capture
+module, checksum, cache, and other effective settings remain available. Its
+`go mod download -modfile=` command overrides an alternate module file selected
+through `GOFLAGS`, preserving ordinary module discovery. Its completed capture
 uses the same ownership transfer when formatting a failure.
 
 ## Configuration discovery