More reliable fix for Go 1.16 build issues (#2119)

* Call "go get -d" when building the a package.

* Fix failing tests

genny/build/apkg.go

@@ -1,6 +1,8 @@
 package build
 
 import (
+	"os/exec"
+
 	"github.com/gobuffalo/genny/v2"
 )
 
@@ -13,10 +15,15 @@ func apkg(opts *Options) (*genny.Generator, error) {
 
 	g.RunFn(copyInflections)
 	g.RunFn(copyDatabase)
+	g.RunFn(addDependencies)
 
 	return g, nil
 }
 
+func addDependencies(r *genny.Runner) error {
+	return r.Exec(exec.Command("go", "get", "-d"))
+}
+
 func copyDatabase(r *genny.Runner) error {
 	defer func() {
 		r.Disk.Remove("database.yml")

genny/build/build_test.go

@@ -56,7 +56,7 @@ func Test_New(t *testing.T) {
 	// we should never leave any files modified or dropped
 	r.Len(res.Files, 0)
 
-	cmds := []string{"go build -tags bar -o bin/foo", "go mod tidy"}
+	cmds := []string{"go get -d", "go build -tags bar -o bin/foo", "go mod tidy"}
 	r.Len(res.Commands, len(cmds))
 	for i, c := range res.Commands {
 		eq(r, cmds[i], c)
@@ -84,7 +84,7 @@ func Test_NewWithoutBuildDeps(t *testing.T) {
 
 		res := run.Results()
 
-		cmds := []string{"go build -tags bar -o bin/foo"}
+		cmds := []string{"go get -d", "go build -tags bar -o bin/foo"}
 		r.Len(res.Commands, len(cmds))
 		for i, c := range res.Commands {
 			eq(r, cmds[i], c)