fmt: fix string interpolation with CastExpr (#9137)

vlib/v/ast/str.v

@@ -162,6 +162,8 @@ pub fn (lit &StringInterLiteral) get_fspec_braces(i int) (string, bool) {
 					} else if sub_expr.left is CallExpr {
 						sub_expr = sub_expr.left
 						continue
+					} else if sub_expr.left is CastExpr {
+						needs_braces = true
 					}
 					break
 				}

vlib/v/fmt/tests/string_interpolation_keep.vv

@@ -11,4 +11,5 @@ fn main() {
 	eprintln('e: $e')
 	_ = ' ${foo.method(bar).str()} '
 	println('(${some_struct.@type}, $some_struct.y)')
+	_ := 'CastExpr ${int(d.e).str()}'
 }