ci: don't include prebuilt libraries in the release

These libraries will be automatically built when needed and cached.

The main reason these were needed is for play.tinygo.org, but I've now
prebuilt them there directly (so they don't need to be built for every
tarball).

builder/musl.go

@@ -12,7 +12,7 @@ import (
 	"github.com/tinygo-org/tinygo/goenv"
 )
 
-var Musl = Library{
+var libMusl = Library{
 	name: "musl",
 	makeHeaders: func(target, includeDir string) error {
 		bits := filepath.Join(includeDir, "bits")