readme: simplify installation instructions

This also cleans up the remaining mentioned LLGOROOT.

cl/compile_test.go

@@ -17,8 +17,6 @@
 package cl_test
 
 import (
-	"os"
-	"path/filepath"
 	"testing"
 
 	"github.com/goplus/llgo/cl"
@@ -67,8 +65,6 @@ func TestPython(t *testing.T) {
 }
 
 func TestGoPkgMath(t *testing.T) {
-	root, _ := filepath.Abs("..")
-	os.Setenv("LLGOROOT", root)
 	conf := build.NewDefaultConf(build.ModeInstall)
 	build.Do([]string{"math"}, conf)
 }