internal/rosa: read-only access to built-ins

This removes potential footguns caused by mutable builtins without requiring unnecessary clones.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/llvm_test.go

@@ -10,7 +10,7 @@ import (
 func TestLLVMInputs(t *testing.T) {
 	const wantInputCount = 470
 
-	_, llvm := rosa.Native().Std().MustLoad(rosa.H("llvm"))
+	_, llvm := rosa.MustLoad(rosa.H("llvm"))
 	var n int
 	for range pkg.Inputs(llvm) {
 		n++