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.

src/utils/python-arch.ts

@@ -0,0 +1,75 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import * as core from "@actions/core";
+import * as log from "./logging";
+
+const execFileAsync = promisify(execFile);
+const PROBE_ARCH = "setup-uv-probe";
+
+export async function setupPythonArch(
+  uvPath: string,
+  pythonArch: string,
+  exportPythonArch: boolean,
+): Promise<void> {
+  if (pythonArch === "") {
+    return;
+  }
+
+  // Older uv releases ignore unknown environment variables. An invalid architecture
+  // confirms that uv recognizes UV_PYTHON_ARCH without finding or downloading Python.
+  const probe = await queryPythonArch(uvPath, PROBE_ARCH);
+  if (probe.exitCode === 0) {
+    throw new Error(
+      "The installed version of uv does not support UV_PYTHON_ARCH. Select a newer uv version or use an architecture-qualified python-version.",
+    );
+  }
+  if (
+    !probe.stderr.includes(
+      `environment variable \`UV_PYTHON_ARCH\` with invalid value \`${PROBE_ARCH}\``,
+    )
+  ) {
+    throw new Error(
+      `Failed to check uv's support for UV_PYTHON_ARCH: ${probe.stderr.trim() || `uv exited with code ${probe.exitCode}`}`,
+    );
+  }
+
+  const selected = await queryPythonArch(uvPath, pythonArch);
+  if (selected.exitCode !== 0) {
+    throw new Error(
+      `Failed to set Python architecture to ${pythonArch}: ${selected.stderr.trim() || `uv exited with code ${selected.exitCode}`}`,
+    );
+  }
+
+  if (exportPythonArch) {
+    core.exportVariable("UV_PYTHON_ARCH", pythonArch);
+    log.info(`Set UV_PYTHON_ARCH to ${pythonArch}`);
+  }
+}
+
+async function queryPythonArch(
+  uvPath: string,
+  pythonArch: string,
+): Promise<{ exitCode: number; stderr: string }> {
+  try {
+    const { stderr } = await execFileAsync(
+      uvPath,
+      ["--no-config", "cache", "dir"],
+      {
+        encoding: "utf8",
+        env: { ...process.env, NO_COLOR: "1", UV_PYTHON_ARCH: pythonArch },
+      },
+    );
+    return { exitCode: 0, stderr };
+  } catch (error) {
+    if (
+      error instanceof Error &&
+      "code" in error &&
+      typeof error.code === "number" &&
+      "stderr" in error &&
+      typeof error.stderr === "string"
+    ) {
+      return { exitCode: error.code, stderr: error.stderr };
+    }
+    throw error;
+  }
+}