Add python-arch support (#1079)

Windows ARM64 jobs may need x64 Python independently of their Python
version pin. Add `python-arch` to export `UV_PYTHON_ARCH` for subsequent
steps and include the selected architecture in the uv and managed-Python
cache keys. The input takes precedence over an inherited environment
value, and unsupported uv versions fail before environment creation.

Requires a uv release with `UV_PYTHON_ARCH` support.

docs/caching.md

@@ -11,6 +11,7 @@ The cache key is automatically generated based on:
   `pc-windows-msvc`)
 - **OS version**: OS name and version (e.g., `ubuntu-22.04`, `macos-14`, `windows-2022`)
 - **Python version**: The Python version in use
+- **Python architecture**: The architecture selected by `python-arch` or `UV_PYTHON_ARCH`, when set
 - **Cache options**: Whether pruning and Python caching are enabled
 - **Dependency hash**: Hash of files matching `cache-dependency-glob`
 - **Suffix**: Optional `cache-suffix` if provided

docs/environment-and-tools.md

@@ -144,6 +144,7 @@ If you want to ignore this, set the `ignore-empty-workdir` input to `true`.
 This action sets several environment variables that influence uv's behavior and can be used by subsequent steps:
 
 - `UV_PYTHON`: Set when `python-version` is specified or the selected `.tool-versions` file contains a supported `python` entry. Controls which Python version uv uses.
+- `UV_PYTHON_ARCH`: Set when `python-arch` is specified. Controls the architecture of Python requests that do not specify one.
 - `UV_CACHE_DIR`: Set when caching is enabled (unless already configured in uv config files). Controls where uv stores its cache.
 - `UV_TOOL_DIR`: Set when `tool-dir` input is specified. Controls where uv installs tool environments.
 - `UV_TOOL_BIN_DIR`: Set when `tool-bin-dir` input is specified. Controls where uv installs tool binaries.
@@ -155,6 +156,7 @@ This action sets several environment variables that influence uv's behavior and
 - `UV_NO_MODIFY_PATH`: If set, prevents the action from modifying PATH. Cannot be used with `activate-environment`.
 - `UV_CACHE_DIR`: If already set, the action will respect it instead of setting its own cache directory.
 - `UV_PYTHON`: If already set and `python-version` is not specified, the action will respect it instead of using the `python` entry from `.tool-versions`.
+- `UV_PYTHON_ARCH`: If already set and `python-arch` is not specified, the action will use it and include it in the cache key. Requires a uv version that supports `UV_PYTHON_ARCH`.
 
 ```yaml
 - name: Example using environment variables