fix!: require NODE_USE_ENV_PROXY for proxy support (#342)

BREAKING CHANGE: Custom proxy handling has been removed. If you use HTTP_PROXY or HTTPS_PROXY, you must now also set NODE_USE_ENV_PROXY=1 on the action step.

tests/post-proxy-requires-native-support.test.js

@@ -0,0 +1,13 @@
+process.env["INPUT_GITHUB-API-URL"] = "https://api.github.com";
+process.env.HTTPS_PROXY = "http://127.0.0.1:3128";
+
+const originalConsoleError = console.error;
+console.error = (...args) => {
+  originalConsoleError(
+    ...args.map((arg) => (arg instanceof Error ? arg.message : arg)),
+  );
+};
+
+await import("../post.js");
+await new Promise((resolve) => setImmediate(resolve));
+process.exitCode = 0;