Use go install instead of go get (#4672)

Calling `go get` was causing `mage check` to fail with:
```
go: modules disabled by GO111MODULE=off; see 'go help modules'
Error: running "go get github.com/elastic/go-licenser@latest" failed with exit code 1
```
This commit fixes it by using `go install` instead.

Author: belimawr

magefile.go

@@ -205,12 +205,12 @@ func (Dev) Package() {
 
 // InstallGoLicenser install go-licenser to check license of the files.
 func (Prepare) InstallGoLicenser() error {
-	return GoGet(goLicenserRepo)
+	return GoInstall(goLicenserRepo)
 }
 
 // InstallGoLint for the code.
 func (Prepare) InstallGoLint() error {
-	return GoGet(goLintRepo)
+	return GoInstall(goLintRepo)
 }
 
 // All build all the things for the current projects.
@@ -560,9 +560,9 @@ func RunGo(args ...string) error {
 	return sh.RunV(mg.GoCmd(), args...)
 }
 
-// GoGet fetch a remote dependencies.
-func GoGet(link string) error {
-	_, err := sh.Exec(map[string]string{"GO111MODULE": "off"}, os.Stdout, os.Stderr, "go", "get", link)
+// GoInstall installs a tool by calling `go install <link>
+func GoInstall(link string) error {
+	_, err := sh.Exec(map[string]string{}, os.Stdout, os.Stderr, "go", "install", link)
 	return err
 }