Use selected module for dependency downloads

preparation.go

@@ -50,8 +50,10 @@ type DependencyPreparationRequest struct {
 
 // PrepareBuildDependencies runs `go mod download` in the selected module with
 // workspace discovery disabled and the locally available compatible Go
-// toolchain selected. It may access module networks and updates the Go module
-// cache using the caller's Go proxy, private-module, credential, and cache
+// toolchain selected. Alternate module files selected by GOFLAGS are overridden
+// for this download; other effective Go settings remain available. It may
+// access module networks and updates the Go module cache using the caller's Go
+// proxy, private-module, credential, and cache
 // settings. If required checksums are missing, the Go command may update
 // go.sum. Cancellation and Go command failures are returned to the caller.
 func PrepareBuildDependencies(ctx context.Context, request DependencyPreparationRequest) error {