dashboard: set GOROOT_BOOTSTRAP for dragonfly-amd64 and freebsd-arm

Updates golang/go#21191

Change-Id: I09854a99c17dec204ee3a1f7c197bb7b69895979
Reviewed-on: https://go-review.googlesource.com/53491
Reviewed-by: Yuval Pavel Zholkover <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

dashboard/builders.go

@@ -147,12 +147,14 @@ var Hosts = map[string]*HostConfig{
 	"host-dragonfly-amd64-tdfbsd": &HostConfig{
 		IsReverse:      true,
 		ExpectNum:      1,
+		env:            []string{"GOROOT_BOOTSTRAP=/usr/local/go"},
 		ReverseAliases: []string{"dragonfly-amd64"},
 		OwnerGithub:    "tdfbsd",
 	},
 	"host-freebsd-arm-paulzhol": &HostConfig{
 		IsReverse:      true,
 		ExpectNum:      1,
+		env:            []string{"GOROOT_BOOTSTRAP=/usr/home/paulzhol/go1.4"},
 		ReverseAliases: []string{"freebsd-arm-paulzhol"},
 		OwnerGithub:    "paulzhol",
 	},