Fix building older versions of go

Set GOROOT_BOOTSTRAP based on GOROOT

cmd.go

@@ -3,6 +3,7 @@ package gvm
 import (
 	"bufio"
 	"io"
+	"os"
 	"os/exec"
 	"sync"
 
@@ -13,6 +14,7 @@ type command struct {
 	Path   string
 	Args   []string
 	Dir    string
+	Env    []string
 	Stdout func(string)
 	Stderr func(string)
 }
@@ -55,6 +57,10 @@ func (c *command) Exec() error {
 	cmd := exec.Command(c.Path, c.Args...)
 	cmd.Dir = c.Dir
 
+	if len(c.Env) > 0 {
+		cmd.Env = append(os.Environ(), c.Env...)
+	}
+
 	var err error
 	var stdout, stderr io.ReadCloser
 	if c.Stdout != nil {

srcrepo.go

@@ -1,6 +1,7 @@
 package gvm
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -123,9 +124,17 @@ func buildGo(log logrus.FieldLogger, buildDir, repo, version, tag string) error
 		return err
 	}
 
+	bootstrap := os.Getenv("GOROOT_BOOTSTRAP")
+	if bootstrap == "" {
+		bootstrap = os.Getenv("GOROOT")
+		if bootstrap == "" {
+			return errors.New("GOROOT or GOROOT_BOOTSTRAP must be set")
+		}
+	}
+
 	if version != "tip" {
 		// write VERSION file
-		versionFile := filepath.Join(tmp, "go", "VERSION")
+		versionFile := filepath.Join(tmp, "VERSION")
 		err := ioutil.WriteFile(versionFile, []byte(version), 0644)
 		if err != nil {
 			return err
@@ -145,6 +154,9 @@ func buildGo(log logrus.FieldLogger, buildDir, repo, version, tag string) error
 	} else {
 		cmd = makeCommand("bash", "make.bash")
 	}
+	cmd.Env = []string{
+		"GOROOT_BOOTSTRAP=" + bootstrap,
+	}
 	return cmd.WithDir(srcDir).WithLogger(log).Exec()
 }