Add option to skip Astral mirror downloads (#387)

## Summary
- add `download-from-astral-mirror` input defaulting to `true`
- skip mirror URL rewriting and download directly from GitHub Releases
when disabled
- document the input and add unit coverage

Fixes #384

Refs: pi-session 019f31bc-bd1c-7276-9b4e-9734fa0aa367

## Testing
- npm run build
- npm run check
- npm run test:unit

action.yml

@@ -23,6 +23,10 @@ inputs:
   manifest-file:
     description: "URL to a custom manifest file in the astral-sh/versions format."
     required: false
+  download-from-astral-mirror:
+    description: "Download Ruff from the Astral mirror instead of directly from GitHub Releases."
+    required: false
+    default: "true"
   github-token:
     description:
       "Used for authenticated downloads of Ruff release artifacts from GitHub."