ci: run the test environment on the WordPress version the plugin requires

`build` fails before any test runs:

    Failed to activate plugin. Current WordPress version (6.4) does not
    meet minimum requirements for Sparxstar Starmus Audio. The plugin
    requires WordPress 6.9.
    Error: No plugins activated.

`starmus-audio-recorder.php` declares `Requires at least: 6.9` while
`.wp-env.json` pinned core to 6.4, so the plugin could never activate in
its own test environment. Not caused by this PR — it is red on main for
the same reason — but #27 already carries this exact one-line fix, and
porting it is cheaper than waiting for that PR to merge. It no-ops once
main has it.

Deliberately not porting #27's EthicalCheck rewrite. That swaps the dead
`apisec-inc/ethicalcheck-action` for a different third-party action, and I
could not verify that the replacement resolves — outbound checks from this
environment return 403 from the proxy, which is not an answer either way.
Shipping an unverified `uses:` into a security workflow is the failure mode
the org rules exist to prevent. #27 authored that change and should carry
it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016cDiLJsQCkPdU4ghzcQSTY

Author: claude

.wp-env.json

@@ -1,5 +1,5 @@
 {
-  "core": "WordPress/WordPress#6.4",
+  "core": "WordPress/WordPress#6.9",
   "phpVersion": "8.2",
   "plugins": ["."],
   "themes": ["https://downloads.wordpress.org/theme/twentytwentyfour.1.0.zip"],