CI: pin playground checkout to adamziel/compile-extension

The wasm-spike workflow needs @php-wasm/compile-extension's source map
and load-built-extension.mjs harness, neither of which exist on
wordpress-playground trunk. They live on the adamziel/compile-extension
branch (PR #3582). Default the checkout there so the resolvePHPRelease
step can read compile.ts and run-spike.mjs can find the harness.

Author: adamziel

.github/workflows/wasm-spike.yml

@@ -16,7 +16,7 @@ on:
       playground-ref:
         description: 'wordpress-playground branch/tag/SHA to build against'
         required: false
-        default: 'trunk'
+        default: 'adamziel/compile-extension'
 
 jobs:
   build-and-load:
@@ -39,7 +39,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: WordPress/wordpress-playground
-          ref: ${{ github.event.inputs.playground-ref || 'trunk' }}
+          ref: ${{ github.event.inputs.playground-ref || 'adamziel/compile-extension' }}
           path: wordpress-playground
 
       - name: Set up Node