opa: Update to 0.48.0

Changes:
## 0.48.0

This release rolls in security fixes from recent patch releases, along with
a number of bugfixes, and a new builtin function.

* Improved error reporting available in `opa eval`: `--show-builtin-errors`
  option added to show all the errors, not just the first one
* New Built-in Function: `time.format`: it is now possible to format a time
  value from nanoseconds to a formatted timestamp string via a built-in
  function.
* Optimization in rule indexing
* Bundle fetching with AWS Signing Version 4A

(leot)