fix: Parse octal echo escapes

Found by this eslint issue:

/puter/packages/phoenix/src/puter-shell/coreutils/coreutil_lib/echo_escapes.js
  107:59  error  'hexchars' is not defined  no-undef

packages/phoenix/src/puter-shell/coreutils/coreutil_lib/echo_escapes.js

@@ -104,7 +104,7 @@ const echo_escapes = {
             caller.output(NUL);
             return;
         }
-        caller.output(String.fromCharCode(Number.parseInt(hexchars, 8)));
+        caller.output(String.fromCharCode(Number.parseInt(octchars, 8)));
     },
     '\\': caller => caller.output('\\'),
 };