internal/encoding: don't interpret file as UTF8 with binary encoding

Using, for example, `@embed(type=binary)` CUE would interpret the file's
contents as UTF8. Since not all byte-sequences are valid UTF8, some will not be
interpretable. Bytes sequences that fail to be properly interpreted will not
raise an evaluation error but instead yield erroneus bytes to the evaluator.

This change makes it so the file's contents will be passed to the evaluator
verbatim by skipping the UTF8 decoder and reading directly from the filereader.

Signed-off-by: nichtsundniemand <rufus.schaefing@gmail.com>

internal/encoding/encoding.go

@@ -262,7 +262,9 @@ func NewDecoder(ctx *cue.Context, f *build.File, cfg *Config) *Decoder {
 		i.err = err
 		i.expr = ast.NewString(string(b))
 	case build.Binary:
-		b, err := io.ReadAll(r)
+		// Binary files should not generally be treated as UTF-8. Don't use the
+		// [transform.Reader] created above but read directly from the file instead.
+		b, err := io.ReadAll(srcr)
 		i.err = err
 		s := literal.Bytes.WithTabIndent(1).Quote(string(b))
 		i.expr = ast.NewLit(token.STRING, s)