Cache the results of Regex.Expand()

### TL;DR

In render_action_regexreplace.go, we call *Regexp.Expand() to expand a string like `hello ${person}` into `hello Alice`. Each `$whatever` represents a regex subgroup.

In some cases we might execute the same expansion (same input string, same subgroup values), and we might be able to save time by caching the results instead running the Expand operation again.

We should wait to do this until we have some evidence that our existing approach isn't fast enough.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_