Send commit SHA to asset library API

Attempting to use the tag fails with:

    {
        "error": "Using Git tags or branches is no longer supported. Please give a full Git commit hash instead, or use the Custom download provider for GitHub Releases downloads."
    }

Use the commit hash instead, as described in the action documentation.

https://phabricator.endlessm.com/T35503

Author: wjt

asset-template.json.hb

@@ -7,7 +7,7 @@
   "cost": "MIT",
   "support_level": "community",
   "download_provider": "GitHub",
-  "download_commit": "{{ context.release.tag_name }}",
+  "download_commit": "{{ env.GITHUB_SHA }}",
   "browse_url": "{{ context.repository.html_url }}",
   "issues_url": "{{ context.repository.html_url }}/issues",
   "icon_url": "https://raw.githubusercontent.com/endlessm/godot-block-coding/{{ context.release.tag_name }}/icon.png"