fix(llama_stack): use far-future date for UV_EXCLUDE_NEWER instead of empty string

Empty string is not a valid uv exclude-newer value. Using 2099-01-01 sets the
cutoff so far in the future that no existing package can be newer than it,
effectively disabling the restriction for this step without breaking uv's
argument parsing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: julian-risch

.github/workflows/llama_stack.yml

@@ -112,7 +112,7 @@ jobs:
           # torch's transitive dep sympy has missing upload dates on the PyTorch wheel index;
           # uv falls back to the request timestamp which triggers the exclude-newer cutoff.
           # Disable exclude-newer for this step only to allow torch resolution.
-          UV_EXCLUDE_NEWER: ""
+          UV_EXCLUDE_NEWER: "2099-01-01"
         shell: bash
         run: |
           set -euo pipefail