builder: move Go version code to goenv package

This is necessary to avoid a circular dependency in the loader (which
soon will need to read the Go version) and because it seems like a
better place anyway.

main_test.go

@@ -73,7 +73,7 @@ func TestCompiler(t *testing.T) {
 		t.Run("ARM64Linux", func(t *testing.T) {
 			runPlatTests("aarch64--linux-gnu", matches, t)
 		})
-		goVersion, err := builder.GorootVersionString(goenv.Get("GOROOT"))
+		goVersion, err := goenv.GorootVersionString(goenv.Get("GOROOT"))
 		if err != nil {
 			t.Error("could not get Go version:", err)
 			return