Use GitHub archive zip for WordPress core in wp-env config

Copilot agents can't download from api.wordpress.org. Use the GitHub
archive zip which is fast (~97s vs ~17min for git clone) and available
inside the Copilot sandbox.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: obenland

.wp-env.json

@@ -1,4 +1,5 @@
 {
+	"core": "https://github.com/WordPress/WordPress/archive/refs/heads/master.zip",
 	"multisite": true,
 	"plugins": [
 		"WordPress/gutenberg",