Move NODE_USE_ENV_PROXY assignment to top of file

Relocated the setting of process.env.NODE_USE_ENV_PROXY to the beginning of main.js for improved clarity and to ensure the environment variable is set before any imports or logic are executed.

main.js

@@ -1,4 +1,5 @@
 // @ts-check
+process.env.NODE_USE_ENV_PROXY = "1";
 
 import core from "@actions/core";
 import { createAppAuth } from "@octokit/auth-app";
@@ -15,8 +16,6 @@ if (!process.env.GITHUB_REPOSITORY_OWNER) {
   throw new Error("GITHUB_REPOSITORY_OWNER missing, must be set to '<owner>'");
 }
 
-process.env.NODE_USE_ENV_PROXY = "1";
-
 const appId = core.getInput("app-id");
 const privateKey = core.getInput("private-key");
 const owner = core.getInput("owner");