allowlist: add astral-sh/setup-uv

- Adds astral-sh/setup-uv to the org-wide allow list;
- Rationale, alternatives, and security assessment are included in the PR body.

Author: junrushao

actions.yml

@@ -833,3 +833,7 @@ vmactions/freebsd-vm:
   05856381fab64eeee9b038a0818f6cec649ca17a:
     expires_at: 2050-01-01
     tag: v1.2.3
+astral-sh/setup-uv:
+  '*':
+    expires_at: 2050-01-01
+    keep: true