Use selected module for dependency downloads

internal/gotool/gotool.go

@@ -97,7 +97,7 @@ func New(ctx context.Context, directory string) (*Tool, error) {
 func PrepareBuildDependencies(ctx context.Context, directory string) error {
 	captured := &combinedWriter{}
 	err := process.Run(ctx, process.Invocation{
-		Args: []string{"go", "mod", "download"},
+		Args: []string{"go", "mod", "download", "-modfile="},
 		Dir:  directory,
 		Env: map[string]string{
 			"GOWORK":      "off",

internal/gotool/gotool_test.go

@@ -159,7 +159,7 @@ func TestPrepareBuildDependenciesPreservesPreparationEnvironment(t *testing.T) {
 	if err := json.Unmarshal(contents, &record); err != nil {
 		t.Fatal(err)
 	}
-	if strings.Join(record.Arguments, " ") != "mod download" || record.Directory != repository {
+	if strings.Join(record.Arguments, " ") != "mod download -modfile=" || record.Directory != repository {
 		t.Fatalf("Go invocation = %#v", record)
 	}
 	if record.Workspace != "off" || record.Toolchain != "local" {