interp: fix reading from external global

This fixes https://github.com/tinygo-org/tinygo/issues/3020.

testdata/embed/embed.go

@@ -20,9 +20,12 @@ var (
 //go:embed a/b/.hidden
 var hidden string
 
+var helloStringBytes = []byte(helloString)
+
 func main() {
 	println("string:", strings.TrimSpace(helloString))
 	println("bytes:", strings.TrimSpace(string(helloBytes)))
+	println("[]byte(string):", strings.TrimSpace(string(helloStringBytes)))
 	println("files:")
 	readFiles(".")
 }

testdata/embed/out.txt

@@ -1,5 +1,6 @@
 string: hello world!
 bytes: hello world!
+[]byte(string): hello world!
 files:
 - a
 - a/b