action.yml: list ucrt64 as a valid architecture value

The previous commits taught the implementation to handle
`architecture: ucrt64`, but `action.yml` is what users (and the GitHub
Actions tooling that renders the marketplace page and inlines the
description into IDE tooltips) actually read. Extend the `architecture`
input description to mention the new axis, and call out the caveat
that only `flavor: full` is wired up end-to-end at this point, since
`build-extra`'s `please.sh create-sdk-artifact` does not yet know about
`--architecture=ucrt64` and there is no UCRT64 asset in the
`ci-artifacts` release of `git-sdk-64`.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

action.yml

@@ -11,7 +11,14 @@ inputs:
     default: 'minimal'
   architecture:
     required: false
-    description: 'The architecture of the SDK: x86_64, i686 or aarch64. Note that "aarch64" only supports the "full" flavor for now.'
+    description: >
+      The architecture of the SDK: x86_64, i686, aarch64, or ucrt64.
+      Note that "aarch64" only supports the "full" flavor for now.
+      "ucrt64" selects the UCRT64 variant of git-sdk-64; while the
+      MINGW64-to-UCRT64 migration is in progress it is cloned from a
+      transitional "ucrt64" branch that will eventually replace "main",
+      at which point "x86_64" itself will materialise a UCRT64 SDK.
+      Only the "full" flavor is supported on the "ucrt64" axis for now.
     default: 'x86_64'
   msys:
     required: false