feat: add proxy support (#102)

Adds support for the following environment variables:

- `https_proxy`
- `HTTPS_PROXY`
- `http_proxy`
- `HTTP_PROXY`
- `no_proxy`
- `NO_PROXY`

tests/main.js

@@ -6,7 +6,7 @@ export const DEFAULT_ENV = {
   GITHUB_REPOSITORY_OWNER: "actions",
   GITHUB_REPOSITORY: "actions/create-github-app-token",
   // inputs are set as environment variables with the prefix INPUT_
-  // https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
+  // https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
   "INPUT_GITHUB-API-URL": "https://api.github.com",
   "INPUT_APP-ID": "123456",
   // This key is invalidated. It’s from https://github.com/octokit/auth-app.js/issues/465#issuecomment-1564998327.