Expose a Python "identity" output (#1036)

https://github.com/pyca/cryptography/pull/15572#discussion_r3913508686
has the context for this: TL;DR our current `python-version` output
mirrors the "request" version exactly, which means that it's
insufficient for any downstream that needs to manage its own cache keys
(since caches shouldn't be shared across release candidates, but the `uv
python` request version doesn't include RC numbers).

The first commit here was my attempt to fix this by exposing the runtime
Python version, but this too is imprecise: the runtime version doesn't
indicate the interpreter variant (e.g. freethreading), which is also
important to capture in the cache identity.

My solution here is to expose `python-runtime-id`, which is just the
`key` of the active Python version from `uv python list
--output-format=json`.

---------

Signed-off-by: William Woodruff <william@yossarian.net>

action.yml

@@ -107,6 +107,8 @@ outputs:
     description: "Path to the activated venv if activate-environment is true"
   python-version:
     description: "The Python version that was set."
+  python-runtime-id:
+    description: "An opaque identifier reported by uv for the activated venv's Python runtime. Empty when activate-environment is false."
   python-cache-hit:
     description: "A boolean value to indicate the Python cache entry was found"
 runs: